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Introduction 
to 



Optical Mineralogy 
and Petrography 



M. G. EDWARDS 



Louis Byrne 

Slichter 




INTRODUCTION 
TO 

Optical Mineralogy 
and Petrography 

The Practical Methods of 

Identifying Minerals 

in Thin Section 

With the 

Microscope 

and 

The Principles Involved in 
The Classification of Rocks 



By 
M. G. EDWARDS, A.M. 

Instructor in Geology and Mineralogy 
Case School of Applied Science. 



CLEVELAND, OHIO, 
1916. 



Copyright, 191G, by 
M. G. EDWARDS 



The Gardner Printing Co. 

Cleveland 

1916 



<3eo/. 
Ub. 



PREFACE. 

IN THE preparation of this volume the writer has at- 
tempted to gather together and systematize in a manner 
accessible for ready reference those facts which are essen- 
tial to a field geologist or to a mining engineer in an 
understanding of the fundamental principles involved in 
the classification and identification of rocks. In the field, 
a preliminary classification is usually made by macro- 
scopic means. However, it is often necessary to make a 
more careful classification by a microscopic examination 
of a thin section of the minerals comprising the rock 
mass. To do this successfully requires a knowledge of 
the application of light to crystalline substances. 

This volume differs from most of the reference and 
text books relating to this subject in that it incorporates 
in one volume the elements of optical mineralogy and the 
elements of petrography. In Part One, eight general 
operations for the determination of unknown minerals in 
thin section are described, prefaced by a short summary 
of the principles of optics which apply to the transmission 
of polarized light through minerals. Descriptions of fifty- 
eight of the most common of the rock-making minerals are 
given, special attention being given to the criteria for 
the determination of these minerals in thin section. 
Their form, cleavage, twinning, color, refringence, bi- 
refringence, extinction angles, pleochroism, absorption, 
optical character, inclusions, alterations, occurrences, 
uses, and differentiation from similar minerals, are all 
discussed whenever applicable. An elementary knowl- 
edge of crystallography and descriptive mineralogy is 
assumed. 



4 OPTICAL MINERALOGY AND PETROGRAPHY 

In Part Two, the principles of petrography are dis- 
cussed briefly. Attention is given to the classification 
and description of the more important igneous rock 
types. 

Following Iddings, Winchell, and other American 
petrographers, the symbols X, Y, and Z, are here em- 
ployed in referring to the axes of ether elasticity, instead 
of the German a, b, and c, used in many text and refer- 
ence books. This is done to avoid confusion, especially 
in conversation or discussion, with the crystallographic 
axes. 

The writer is indebted to Professor Frank R. Van 
Horn for suggestions. Among the reference and text 
books most frequently consulted the writer wishes to 
acknowledge Winchell's "Elements of Optical Miner- 
alogy," Johannsen's "Manual of Petrographic Methods," 
Luquer's "Minerals in Rock Sections," Rogers's "Study of 
Minerals," Findlay's "Igneous Rocks," Kemp's "Hand- 
book of Rocks," Ries' and Watson's "Engineering Geol- 
ogy," and Farrell's "Practical Field Geology." 

M. G. EDWARDS. 
Cleveland, Ohio, February, 1916. 



TABLE OF CONTENTS 

INTRODUCTION. PAGE 

PART ONE. OPTICAL MINERALOGY. 

CHAPTER 1. THE ELEMENTS OF OPTICS AND THE 
APPLICATION OF POLARIZED LIGHT TO CRYSTALLINE 

SUBSTANCES 13 

The Nature of Light Isotropic and Aniso- 
tropic Media Uniaxial and Biaxial Crystals 

Index of Refraction Double Refraction 
Interference Polarization. 

CHAPTER 2. THE POLARIZING MICROSCOPE AND ITS 
PARTS 25 

Microscope Nicol Prisms Condensing Lens 

Cross Hairs Stage Mirror Objective 
Bertrand Lens Ocular Micrometer Ad- 

justment Screws. 

CHAPTER 3. GENERAL METHODS OF MINERAL DE- 
TERMINATION 33 

1. By the General Physical Properties; 2. By 
the Relative Refractive Index Method of Due 
de Chaulnes Immersion Method Becke 
Method Scale of Refringence ; 3. By the Bire- 
fringence Interference Colors Axes of 
Ether Vibration Optic Plane Scale of Bi- 
refringence. 

CHAPTER 4. GENERAL METHODS OF MINERAL DE- 
TERMINATION (Continued) 51 

4. By Axial Interference Figures Uniaxial 



6 OPTICAL MINERALOGY AND PETROGRAPHY 

and Biaxial; 5. By Dispersion; 6. By Optical 
Character Quartz-Sensitive Tint Quarter- 
Undulation Mica Plate Quartz Wedge ; 7. By 
the Extinction Angle Parallel and Oblique 
Extinction; 8. By Pleochroism. 

CHAPTER 5. DESCRIPTION OF IMPORTANT ROCK- 
MAKING MINERALS 69 

Isometric Minerals. 

CHAPTER 6. DESCRIPTION OF MINERALS (Con- 
tinued) 75 

Tetragonal Hexagonal. 

CHAPTER 7. DESCRIPTION OF MINERALS (Con- 
tinued) 86 

Orthorhombic Monoclinic Triclinic. 

PART TWO. PETROGRAPHY. 

CHAPTER 8. GENERAL DISCUSSION OF IGNEOUS 

ROCKS 123 

Classification Essential and Accessory Min- 
erals Primary and Secondary Minerals 
Texture Rosenbusch's Law Volcanic and 
Plutonic Rocks Petrogeny Magmas Dif- 
ferentiation Magmatic Stoping Crystal- 
lization Influence of Gases on a Magma 
Relation between Composition of Igneous Rocks 
and Magmas Aids in the Determination of 
Igneous Rocks in Hand Specimen. 

CHAPTER 9. IGNEOUS ROCK TYPES PLUTONIC 

ROCKS 138 

Granite Syenite Nephelite and Leucite 
Syenite Diorite Gabbro and Norite Es- 



TABLE OF CONTENTS 7 

sexite Theralite, Shonkinite, Malignite, Ijol- 
ite, Missourite Peridotite Pyroxenite, 
Hornblendite. 

CHAPTER 10. IGNEOUS ROCK TYPES VOLCANIC 

ROCKS ; 158 

Rhyolite Trachyte Phonolite Andesite 
Dacite Basalt Trachydolerite Tephrite, 
Basanite Leucitite, Nephelinite Limburg- 
ite Augitite. Pyroclastic rocks. 

CHAPTER 11. SEDIMENTARY AND METAMORPHIC 

ROCKS 172 

Sedimentary Rocks Classification Conglom- 
erate Breccia Sandstone Shale Loess 

Sand Dunes Limestone Gypsum An- 
hydrite Halite Flint Iron Ores Phos- 
phate Rock Carbonaceous Rock. 
Metamorphic Rocks Composition, Chemical 
and Mineralogical Agents of Metamorphism 

Gneiss Schist Quartzite Slate and 
Phyllite Marble Serpentine Ophicalcite 

Soapstone. 

APPENDIX. 

SUGGESTIONS FOR GEOLOGICAL WORK 185 

Observation for Geological Mapping. Criteria 
of Relative Age. Table for the Examination of 
Rocks in the Laboratory. 

INDEX . . 193 



INTRODUCTION. 

THE TERM Petrology is derived from the two Greek 
words petros (rock) and logos (discourse), from which 
the modern definition, the science or treatise of rocks, has 
been evolved. The term has a wide scope, and embraces 
not only the study of the origin and transformation of 
rocks but a consideration of their mineral composition, 
classification, description and identification based upon 
either megascopic or microscopic characteristics. 

Petrology may be subdivided into the following spe- 
cial studies : 

Petrogeny, which is concerned with the origin of 
rocks, and 

Petrography, which is concerned with the systematic 
classification and description of rocks megascopically and 
microscopically. It is the latter phase of the subject 
which is dealt with chiefly in the following notes. 

Petrography may be divided for the sake of con- 
venience into megascopic petrography and microscopic 
petrography, depending upon whether or not the student 
is basing his identification, classification and description 
upon a study of the rock in hand specimen or in thin sec- 
tion with the aid of the polarizing microscope. 

The use of the polarizing microscope necessarily en- 
tails a brief review of the elements of optics and a con- 
sideration of the application of polarizing light to crys- 
talline substances. This is a special study in itself, and 
is called Optical Mineralogy. Assuming that the student 
has had little or no previous experience with the study 
of the optical properties of minerals, a short review of 
the optical characters of the more important rock-making 



10 OPTICAL MINERALOGY AND PETROGRAPHY 

minerals is given. Special attention is given to the 
criteria for the determination of the mineral in thin sec- 
tion and diagnostics for the differentiation of the mineral 
from similar minerals. 

History of Petrography. Great advances in the knowl- 
edge of mineralogy marked the latter half of the eight- 
eenth century. Incidentally there followed several at- 
tempts to classify rocks, which resulted in 1787 in the 
publication of two classifications by Karl Haidinger 
(Vienna) and A. G. Werner (Dresden). Werner's classi- 
fication was stratigraphic rather than petrographic, but 
he described rocks in terms of mineralogical composition 
and physical characteristics, and he differentiated be- 
tween essential and accessory minerals. 

In 1801, Abbe R. J. Hauy (Paris), a mineralogist, 
published the first systematic classification, and his 
"Traite de mineralogie" with subsequent revisions re- 
mained a classic for a long period. He distinguished five 
classes of rocks: stony and saline, combustible nonme- 
tallic, metallic, rocks of an igneous or aqueous origin, 
and volcanic rocks. 

John Pinkerton (England) in 1811 published a "Pe- 
trology, a Treatise on Rocks," of 1200 pages. In view of 
the fact that natural history was divided into three king- 
doms the animal, vegetable, and mineral he believed 
it the most natural classification to subdivide the mineral 
kingdom into provinces and domains. Accordingly he 
introduced the following three provinces : Petrology, the 
knowledge of rocks or stones in large masses ; Lithology, 
the knowledge of gems and small stones, and Metallogy, 
the knowledge of metals. Pinkerton's volume was lightly 
regarded even by his contemporaries. 

Cordier (France) in 1815 classified rocks as feld- 
spathic or pyroxenic, and made subdivisions according to 
texture. 



INTRODUCTION 11 

Karl von Leonhard (Heidelberg) in 1823 and Alex- 
andre Brongniart in 1827 proposed systems which mark 
the real origin of systematic petrography. Mineral com- 
position was the chief factor in the classification. The 
former established four divisions : heterogeneous rocks, 
homogeneous rocks, fragmental rocks, and loose rocks. 
The latter made only two classes: homogeneous rocks 
and heterogeneous rocks. 

Hermann Abich in 1841 made a classification of the 
eruptive rocks according to the content of the various 
feldspars. 

The term petrography was perhaps first used by Carl 
Friedrich Naumann, who in 1850 published his "Lehr- 
buch der Geognosie," in which he divided all rock classes 
into crystalline rocks, clastic rocks, and rocks which are 
neither crystalline nor clastic. In a later revision he 
recognized only two classes: the original, and the de- 
rived. 

Several classifications were presented in the next few 
decades by von Gotta (1855), Senft (1857), Blum 
(1860), Roth (1861), Scheerer (1864), Ferdinand Zirkel 
(1866), and F. von Richthofen (1868), based upon min- 
eral constitution, chemical composition, structure, and 
texture, with an increasing tendency to emphasize the 
importance of mineral composition. 

A new era in the development of petrography dawned 
with the introduction of the polarizing microscope. With 
the greater knowledge of mineral composition and texture 
thus revealed, the old schemes were discarded or radically 
revised, new terms introduced, and the nomenclature 
became rapidly more complex. Although Henry Clifton 
Sorbey (England) perhaps first used the microscope in 
the determination of rock sections, it was not until the 
decade between 1870 and 1880 that microscopic methods 
began to exert a controlling influence in the development 



12 OPTICAL MINERALOGY AND PETROGRAPHY 

of the science. Zirkel in 1873 produced "Die mikroskop- 
ische Beschaffenheit der Mineralien und Gesteine," which 
shows a remarkable and significant advance in the prog- 
ress of petrography in the eight years following the pub- 
lication of his "Lehrbuch." He dealt chiefly with feld- 
spathic, massive, composite, and nonclastic rocks. 

In France in 1879 the "Mineralogie micrographique," 
by F. Fouque and A. Michel Levy, appeared. Rock classi- 
fication was based upon the mode of formation, the geo- 
logical age, and the specific mineral properties, which 
includes the nature of the mineral and the structure of 
the rock. 

Subsequent editions of the original works of Rosen- 
busch and Zirkel, and a number of new noteworthy con- 
tributions by Roth (1883), Teall (1888), Loewinson- 
Lessing (1890-1897), and Johannes Walther (1897) 
appeared, chief attention being given to the classification 
of igneous rocks on the basis of origin, age, and char- 
acters. 

"\Vithin the last twenty years a number of American 
petrographers have made noteworthy contributions to 
the science of rock classification, and with the coopera- 
tion of the field geologist who has gradually become more 
and more painstaking in the matter of collecting and 
labeling rock specimens for future study, they hope to 
evolve from the present classification which is marred 
by a complexity of nomenclature, a logical and compre- 
hensive system of classification which will approach in 
construction as closely as possible a truly natural arrange- 
ment. 

Among the earlier American petrographers who made 
valuable contributions toward the development of the 
science are J. F. Kemp, J. S. Diller, Whitman Cross, J. P. 
Iddings, and F. D. Adams. 



THE ELEMENTS OF OPTICS 13 



PART ONE. OPTICAL MINERALOGY. 

CHAPTER 1. 

The Elements of Optics, and the Application of Polarized 
Light to Crystalline Substances. 

The Nature of Light. Light is a form of energy 
which in a homogeneous medium as the ether is trans- 
mitted in a rapid wave motion in straight lines with no 
change in the direction of propagation. This wave mo- 
tion is considered to be a resultant of simple harmonic 
motion and a uniform motion at right angles to this. In 
other words, wave motion is a vibration which takes 
place at right angles to the direction of propagation of 
the light. 

A ray of light is a line which designates the direction 
of transmission of the wave. The intensity of light de- 
pends upon the rate or wave-length of the vibrations. 
Color sensation is determined by the number of waves 
of light which reach the eye in a given time. The wave- 
length for red light is 760 millionths of a millimeter, 
and the wave-length for violet light is 397 millionths of 
a millimeter. White light is the sum of light of all these 
various wave-lengths. The velocity of light of all colors 
in vacuo is the same, and is about 300,000 km per second. 

Isotropic Media. Light is transmitted with equal 
velocity in all directions in certain media, as air, water, 
and glass. Light which is transmitted through such a 
medium if it finds its source in that medium will be propa- 
gated as spherical waves, in which the wave-front or 



14 OPTICAL MINERALOGY AND PETROGRAPHY 

wave-surface forms a continuous surface, and all points 
on that surface are equidistant from the source. A ray 
of light is perpendicular to its wave-front. 

In an isotropic substance, this wave-surface may be 
represented by the surface of a sphere. Any plane pass- 
ing through this imaginary sphere in any position will 
have a circular outline. Gases, liquids, amorphous sub- 
stances as volcanic glass, and crystals of the isometric 
system, are isotropic substances. The velocity of trans- 
mission of light through these substances is independent 
of the direction of vibration. 

Anisotropic Media. In anisotropic media (as op- 
posed to isotropic media) , the velocity with which light 
is propagated varies with the direction. All substances 
which are not amorphous or which do not belong to the 
isometric system are optically anisotropic. 

Anisotropic crystals are divided into uniaxial and 
biaxial crystals. 

Uniaxial Crystals. In uniaxial crystals, only one 
direction exists in which there is no double refraction of 
light. This is in the direction of the vertical crystallo- 
graphic axis, which is called the optic axis. It lies in the 
direction of either the greatest or least ease of vibration. 
The wave-front which represents the optical structure 
of uniaxial crystals is an imaginary spheroid of revolu- 
tion in which the optic axis is the axis of revolution. A 
plane passing through the spheroid in any direction at 
right angles to the optic axis has a circular outline. Any 
other section has an elliptical outline. Tetragonal and 
hexagonal crystals are uniaxial. 

Biaxial Crystals. In biaxial crystals there are two 
directions corresponding in character to the one optic 
axis of uniaxial crystals, which gives rise to the term 
biaxial. The wave-front which represents the optical 
structure of biaxial crystals is an imaginary ellipsoid 



THE ELEMENTS OF OPTICS 15 

with three unequal rectangular axes. A plane passing 
through this ellipsoid in any direction at right angles to 
either of the optic axes has a circular outline. Any other 
section has an elliptical outline. Orthorhombic, mono- 
clinic and triclinic crystals are biaxial. 

Index of Refraction. The previous' discussion has 
been concerned with light which has. passed through 
homogeneous media. If a system of light waves of the 
same wave-length passes obliquely from one medium into 
another, there will be a change in the direction of trans- 
mission depending upon the relative ease or difficulty 
with which the light may penetrate the new medium. If 
the second medium, such as glass, is optically more dense 
than the first medium, such as air, that portion of the 
wave-front which first strikes the glass will experience 
a greater difficulty in transmission, and its velocity will 
be reduced, while the remainder of the wave-front is still 
traveling with the same velocity in the air. When this 
portion of the wave-front finally reaches the glass, it has 
gained upon the first portion, with a result that the wave 
will have suffered a deflection from its original course. 
From this position the various portions of the wave-front 
continue through the glass with equal velocities. 

This phenomenon is called refraction. It is a change 
of direction at the bounding surface. Refraction is 
toward the perpendicular (to the bounding surface) 
when the passage of a light ray is from the rarer to 
the denser medium, and away from the perpendicular 
in the opposite case. 

In Fig. 1, D C is the bounding surface between two 
media, of which the lower is optically denser than the 
upper. G H is a perpendicular to the bounding surface. 
Angle i is the angle of incidence and angle p is the angle 
of refraction. A constant relation exists between the 
sines of these angles regardless of the direction of trans- 



16 OPTICAL MINERALOGY AND PETROGRAPHY 

mission, which may be expressed as follows: the sine 
of the angle of incidence bears a constant ratio to the 
sine of the angle of refraction. This ratio may be 

sin i 

expressed by the equation - = n, in which n is 

sin r 

the index of refraction and is inversely proportional to 

c 

^~~~ 

F 




Fig;. 1. Reflection and single refraction. 
(Winchell.) 

the wave velocity. In this formula there are two lim- 
iting relations to be considered. If i 0, r = 0, in 
which case the angle of refraction becomes zero. Thus, 
by perpendicular incidence, the ray proceeds in the 
second medium without any change in direction. If 

i = 90, = n or sin r = -. This value of r is known 

sin r n 

as the critical angle, or angle of total reflection, and may 



THE ELEMENTS OF OPTICS 17 

be defined as that angle beyond which no light passes 
from denser to rarer media. All light may pass from 
a rarer to a denser medium, but the amount of light 
which may pass from the denser to the rarer medium 
is limited by the critical angle. The critical angle is 
a constant for the substance. 
Thus for water n = 1.335. 

1 

sin r = 

1.335. 

r = 48 35'. 
And for diamond, n = 2.42. 

sin r = 

2.42 

r = 24 25'. 

If light is to pass from water into air, the rays must 
strike the surface at angles less than 48 35', whereas 
if light is to pass from diamond into air, the rays must 
strike the surface at angles less than 24 25'. Evidently 
more light can enter the diamond than can directly es- 
cape, and this fact is responsible for the brilliancy of 
the gem. The greater the index of refraction, the smaller 
will be the critical angle. In a cut diamond, the facets 
are arranged so that most of the light is totally reflected. 

Most substances have a value for n ranging be- 
tween 1 and 2. The following table gives the indices of 
refraction for a variety of substances : 

Ice . . . . 1.310 Quartz .... 1.547 

1.601 
. 1.75 

1.814 
. 1.952 

2.369 
. 2.429 

2.712 
. 3.016 



Water . 


. 1.335 


Calcite 


, 


Alcohol 


1.36 


Methylene iodide 


Fluorite 


. 1.434 


Garnet 




Common glass . 


1.435 


Zircon . 




Olive oil 


. 1.47 


Sphalerite 




Canada balsam . 


1.536-1.549 


Diamond 




Rock salt 


. 1.544 


Rutile 




Bromoform 


1.59 


Pyrargyrite 


. 



18 OPTICAL MINERALOGY AND PETROGRAPHY 

An adamantine luster is characteristic of minerals 
with an index of refraction above 1.9. 

Ordinarily, the index of refraction of a substance is 
determined by passing the incident ray into the sub- 
stance from air, but other media than air might be used. 
The index of refraction of the substance in air is the 
product of its index in the medium by the index of 
that medium in air. The index of refraction of air 
when referred to a vacuum is 1.000294. 

Elementary phenomena in refraction, such as the ap- 
parent bending of a stick of wood when partially sub- 
merged in water, were no doubt observed in early times. 
The constant ratio between sin i and sin r was first 
established by Descartes in 1637, but it was not until 
Newton succeeded in producing a colored spectrum by 
a prismatic decomposition of white light that the full 
importance of n was realized. 

Double Refraction. Double refraction is the prop- 
erty possessed by all anisotropic crystals of resolving 
a light ray into two rays polarized at right angles to 
each other and traveling in different directions. This is 
due to the fact that upon entering the anisotropic me- 
dium the vibrations of light are made to conform to 
the molecular structure of the medium. In other words, 
light travels with different velocities in different crys- 
tallographic directions in the same substance. 

The ray advances with the greatest velocity when 
it is vibrating parallel to the direction of the greatest 
ease of vibration, and with least velocity when vibrat- 
ing parallel to the direction of least ease of vibration. 
These rays obviously have different indices of refrac- 
tion. The ray which follows the usual laws of single 
refraction is called the ordinary ray, expressed by 0. 
The other ray is called the extraordinary ray (E) be- 



THE ELEMENTS OF OPTICS 



19 



cause it does not follow the usual laws of single re- 
fraction. 

When a ray of light enters an anisotropic medium 
perpendicular to its surface, the ordinary ray passes 
through without suffering refraction, provided the sur- 




l'"ig. 2. Plane wave advancing: perpendicular 
to the vertical axis, showing ether 
vibration and retardation of 
the O and E rays. 
(Winchell.) 

face through which it emerges is parallel to the sur- 
face through which it enters. The extraordinary ray 
is diverted. To this rule the following exceptions must 
be noted. If the original ray enters a substance per- 
pendicular to the surface and at the same time par- 



20 



OPTICAL MINERALOGY AND PETROGRAPHY 



allel to an optic axis, there is no refraction nor polar- 
ization. If perpendicular to the surface and to an optic 
axis, there is no refraction but there is a division into two 
rays traveling with different velocities and polarized at 
right angles to each other. 




Fig. S. Oblique incidence on a gurface 
parallel to the optic axis. (Winchell.) 

When the two rays emerge from the substance, they 
resume parallelism, but the waves of one are slightly 
in advance of the waves of the other. Such waves are 
said to interfere with each other, producing light of 
different colors. Upon this phenomenon is based much 



THE ELEMENTS OF OPTICS 21 

of the work done in examining minerals in thin section 
under the microscope in parallel polarized light. 

Interference. Two waves of like length and ampli- 
tude, if propagated in the same direction and meeting 
in the same phase, unite to form a wave of double am- 
plitude. If these waves differ in phase by half a wave- 
length or an odd multiple of this, they interfere in such 
a way as to extinguish each other. For other relations 
of phase falling between these extreme cases they also 
interfere with each other, forming a new resultant wave, 
differing in amplitude'from each of the component waves. 
We are assuming here the use of monochromatic light 
waves, or light waves of like length. If ordinary white 
light is employed, the waves in case of interference will 
be indicated by the appearance of the colors of the 
spectrum. 

Polarization. Ordinary light is propagated by trans- 
verse vibrations of the ether which take place in all 
directions about the line of propagation. Plane polar- 
ized light is propagated by ether vibrations which take 
place in one plane only. This phenomenon is called polar- 
ization. It may be described as a change in the char- 
acter of reflected or transmitted light, which diminishes 
its power of being further reflected or transmitted. 

Light is polarized by reflection, by single refraction, 
and by double refraction. The plane of polarization of 
light polarized by reflection is defined as the plane con- 
taining the incident and the reflected rays, the vibra- 
tions taking place at right angles to it. The plane of 
polarization of the refracted ray is the plane at right 
angles to the vibration direction, consequently at right 
angles to the plane of the incident and the reflected rays. 
That light is polarized when reflected may be shown 
experimentally by the use of two reflecting surfaces. 

Nicol Prism. The Nicol prism, so named after its 



22 OPTICAL MINERALOGY AND PETROGRAPHY 

inventor, Nicol, is a device for producing polarized light. 
It consists of a clear transparent crystal of calcite known 
as Iceland spar, as it is obtained almost exclusively 
from caves in certain basalts in Iceland. The vertical 
faces are natural cleavage faces, in which the end cleav- 
ages, inclined 71 degrees to the obtuse edges of the 
prism, are ground down and polished so as to make 
an angle of 68 degrees with the obtuse vertical edges. 
It is then cut diagonally in two parts perpendicular to 





fig. 4. Side view of 
the Nicol prism. 



Fig. 5. End view of 
the Nicol prism. 



the short diagonal of the end face. The two parts are 
cemented together in their original position by Canada 
balsam, a resin obtained from a species of fir. It has 
an index of refraction of 1.536. Since calcite is a doubly 
refracting substance, the Nicol prism refracts a ray of 
light into two rays the ordinary ray, having an index 
of 1.658, and the extraordinary ray, having an index 
of 1.486. 

The angle at which the two new planes are polished, 
as well as the angle at which the crystal is cut, are 
so calculated that the ordinary ray will strike the bal- 



THE ELEMENTS OF OPTICS 23 

sam at an angle greater than the critical angle. Conse- 
quently, the ordinary ray is totally reflected and is ab- 
sorbed in the blackened walls of the cork mountings. 
The extraordinary ray passes through the balsam as a 
completely polarized ray which is vibrating in a known 
direction, namely, parallel to the short diagonal of the 
calcite rhomb. When two Nicol prisms have their short 
diagonals parallel, light passes through without being 
changed except for a decrease in intensity. If one of the 
nicols is revolved, the light gradually diminishes until the 
nicols are at 90 degrees to each other, when darkness 
results. 



24 OPTICAL MINERALOGY AND PETROGRAPHY 



CHAPTER 2. 
The Polarizing Microscope and Its Parts. 

The Polarizing Microscope. In order to ascertain 
the peculiarities of minerals of each of the crystallo- 
graphic systems as they are manifested in polarized 
light, the polarizing microscope is used. This instru- 
ment is applicable to the study of the form, optical prop- 
erties, and mutual relations of the minerals as they are 
found in thin sections of rocks, making it a valuable 
aid to geological research. It is likewise used to great 
advantage in the study of small isolated crystals, or 
fragments of crystals. A determination of the follow- 
ing characteristics of the unknown mineral is of 
particular value in its identification: crystal form as 
shown in outline, direction of cleavage lines, refrac- 
tive index, light absorption in different directions, the 
isotropic or anisotropic character, position of the axial 
plane and the nature of the axial interference figures, 
the strength and character (positive or negative) of 
the double refraction, presence and nature of inclusions, 
type of twinning. 

In addition to the parts essential to the ordinary 
microscope the polarizing microscope contains the fol- 
lowing parts: two Nicol prisms, a lens for convergent 
polarized light, a rotating stage, and an ocular with cross 
hairs. 

Nicol Prisms. The effects due to polarized light can- 
not usually be distinguished except by a combination of 
two Nicol prisms. The upper nicol is not revolvable, 



THE POLARIZING MICROSCOPE AND ITS PARTS 25 




fi. The Fiiesa microscope. 



26 OPTICAL MINERALOGY AND PETROGRAPHY 

and is placed in a support between the ocular and the 
objective. It can be pushed in or out of the tube at will. 
It is called the analyzer. 

The lower nicol, which is revolvable, is placed be- 
neath the stage. For ordinary work, its principal sec- 
tion (i.e., its shorter diameter) is placed at right angles 
with the principal section of the upper nicol. It may be 
raised or lowered without disturbing the centering. This 
nicol is called the polarizer. 

The principal section of the analyzer is left and right, 
and that of the polarizer is front and rear. In this posi- 
tion the field is dark and the nicols are "crossed." 

When a thin section is examined over the lower 
nicol or between two nicols without the convergent light, 
it is said to be done in parallel polarized light. When 
the lower nicol is used alone, its vibration plane must 
be known. A simple test is to place a cleavage frag- 
ment of calcite within the field of view. It has a high 
relief when its long diagonal is parallel to the plane 
of vibration of the nicol. 

A section of biotite cut at right angles to its cleav- 
age has its greatest absorption when its cleavage direc- 
tion is parallel to the plane of vibration of the polar- 
izer. Consequently, it is darkest in this position. Tour- 
maline, on the other hand, extinguishes vibrations at 
right angles to the optic axes, i.e., it absorbs the ordi- 
nary ray, and only the light rays vibrating parallel to 
the crystallographic axis c emerge. 

By removing the Nicol prism from the tube, the 
separating plane of the balsam along which the two 
fragments of calcite were cut may be seen upon looking 
through the prism at an angle. The vibration direc- 
tion of the ray which passes through the prism (the ex- 
traordinary ray) is normal to the layer of balsam. 

The polarizer may be removed from the microscope 



THE POLARIZING MICROSCOPE AND ITS PARTS 27 

and light reflected from a horizontal surface, such as 
a plate of glass or a table top, examined through it. Since 
light is polarized in a plane parallel to the reflecting 
surface, the polarizing plane of the nicol lies at right 
angles to the reflecting surface when the latter appears 
dark. 

Lens for Convergent Light. When the operation de- 
mands convergent light, a powerful convergent lens can 
be thrown into the tube of the microscope over the 
polarizer by means of a lever beneath the stage. This lens 
may be raised with the lower nicol until the surface 
of the lens is practically in contact with the glass slide 
holding the thin section. 

The Rotating Stage. The stage is a circular table 
upon which the thin section is placed for examination. 
The edge has a graduated scale and vernier reading to 
minutes. The center of the stage must coincide with 
the optical center of the tube. Centering is done by 
means of two centering screws, 90 degrees apart, lo- 
cated on the lower end of the tube. The thin section is 
held in place by two spring object clips. Recent forms 
of microscopes are equipped with mechanical stages 
which have freedom of movement in a left-and-right 
direction and in a front-and-rear direction, thus allowing 
a rapid inspection of every part of the section. 

Gross Hair.-^Cross hairs are placed in the ocular 
at right angles to each other, one running left and right 
and the other front and rear, in agreement with the 
principal sections of the nicols when crossed. 

To measure a plane angle in thin section, or the in- 
terfacial angle of a small, flat crystal, the stage is cen- 
tered with the intersection of the two edges at the cen- 
ter of the cross hairs. A reading is made when one 
edge of the crystal is parallel to the left-and-right cross 
hair, then the stage is revolved until the other edge is 



28 OPTICAL MINERALOGY AND PETROGRAPHY 

parallel to the same cross hair but on the opposite side 
of the center. Another reading is taken. The difference 
between the two readings is the external angle. 

Other parts of the microscope which deserve expla- 
nation and suggestions as to proper use are taken up in 
order. 

The Mirror. The mirror which is attached to the 
substage reflects light from the source to the object. 
A plane mirror forms one side and a concave mirror 
the other. The former is used for low magnification, 
where a weak light is sufficient. The latter is used for 
higher magnifications. This mirror concentrates the 
light by converging the rays included within an angular 
aperture of about 40 degrees. For still higher mag- 
nifications and for all phenomena observed in convergent 
light the condensing lens is used. 

A proper use of the mirror is essential to the most 
efficient use of the microscope. When parallel rays, such 
as ordinary daylight, are used, they are reflected from 
the mirror with a slight loss of intensity. They are re- 
flected from the concave mirror with increased intensity, 
the rays coming together at the focal point. If the 
source of light is close to the instrument, the focal length 
is larger. To meet this adjustment, the mirror is at- 
tached to a sliding vertical bar. Since the condensing lens 
has its focus some distance above its upper surface, the 
plane mirror is used in connection with it. 

The Objective. Objectives are classified according to 
their magnification. An objective of low power has a 
focal length above 13 mm and a magnification less than 
15 diameters; it is of medium power when its focal 
length is between 12 and 5 mm and its magnification 
is 40 diameters; of high power when its focal length 
is less than 4.5 mm and its magnification exceeds 40 
diameters. The objectives most commonly used are 



THE POLARIZING MICROSCOPE AND ITS PARTS 29 

numbers 3 and 7, the former for searching out an object 
and for making the preliminary examination, and the 
latter for convergent light and high power. 

A thin section may be considered as made up of a 
series of planes superimposed one above the other, only 
one of which may be seen for one adjustment of focus. 
With low-power objectives one can see objects lying in 
slightly different planes, but with high-power lenses this 
is impossible, as the depth of focus diminishes inversely 
as the numerical aperture. The brightness of the image 
increases as the square of the numerical aperture. 

Resolving Power. The resolving power of an ob- 
jective is that property by virtue of which one is able 
to see the finer details of an object. This resolving power 
increases with the number and obliquity of the rays 
coming from the object, consequently an immersion fluid 
by increasing the number of rays brought to the object 
increases the resolving power. In petrographic work 
no very great magnifying powers are required, and im- 
mersion lenses are not much used except for particular 
kinds of work. 

When two points are removed from the eye 6,876 
times the distance separating them they will appear 
as a single point. The eye is able to distinguish only 
about 250 lines to an inch. Thus pleurosigma angulatum 
with about 50,000 lines to the inch can be resolved by a 
one-half inch objective so as to be clearly seen with a 
three-quarters inch ocular but not with one-and-one-half 
inch. A much smaller line may be seen than the inter- 
val between two lines. 

Cost of Objectives. Objectives with a focal length 
of 25 mm and over cost about $4 each; between 25 and 
10 mm, $5.50 to $10; 10 to 3 'mm, $7 to $15; 3 to 2 mm, 
about $20. Students are urged to treat them with care. 



30 OPTICAL MINER*u<)GY AND PETROGRAPHY 

Bertrand "Lens. In the center of the microscope tube 
above the analyzer is the Bertrand lens, which may be 
thrown in or out of the tube by means of a sliding car- 
rier. It acts as a small microscope which is used with 
the ocular to magnify interference figures. 

The Ocular. The Huygens ocular which is most gen- 
erally used in petrographic microscopes consists of two 
simple plane-convex lenses placed with their plane sur- 
faces toward the eye. The upper lens is known as the 
eye lens and the lower as the collective or field lens. The 
focal length of the eye lens is one third of the field 
lens, and they are separated a distance equal to the sum 
of their focal lengths. The rays of light emerging from 
the eye lens are parallel and thus cause the eye less 
fatigue. 

The cross hairs which are placed in the eyepiece are 
made of spider web, -the dark thread from the inside 
of the nest being the best. 

Micrometer. It is desirable at times to measure 
small distances such as the dimensions of small crystals. 
A special eyepiece called the micrometer has been devised 
for this purpose. It contains a scale etched on glass. On 
the stage of the microscope a scale reading to hundredths 
of a mm is placed. It is then necessary to find to how 
many hundredths of a mm each division of the eyepiece 
is equivalent. 

It may be well for the student, in order to become 
familiar with the use of the micrometer, to construct 
a table showing the value of the ocular micrometer for 
each objective. The stage micrometer is used for this 
purpose. 

Adjustment Screws. The tube carrying the eyepiece 
and objective has a fine adjustment screw, the edge of 
which is graduated. It moVes against a fixed index at- 
tached to the tube, by which means the distance through 



THE POLARIZING MICROSCOPE AND ITS PARTS 31 

which the tube is raised or lowered can be measured 
to .001 mm. 

The student is advised as a laboratory illustration 
to determine the amount which one revolution of the 
fine adjustment screw raises the objective. To do this, 
measure the thickness of a glass plate or cover glass by 
focusing carefully on the lower surface of the glass and 
then upon the upper surface. This distance is meas- 
ured by the micrometer by setting the glass plate on edge, 
slightly embedded in paraffin or wax, or supported other- 
wise. 

Use of the Microscope. The best light for micro- 
scopic work is that coming from the north; the next 
best from the east. Direct sunlight should never be used. 
The table should be firm, and of a height to suit the 
convenience of the individual. The instrument should 
be placed directly in front of the observer, so that both 
hands can be used for manipulation. 

The eye which is not used for observation should also 
be kept open. Although it may seem difficult at first 
to concentrate the gaze on the thin section, it will be 
found to be far less fatiguing. When using high powers, 
the eye must be kept very close to the ocular, with low 
powers slightly farther removed. When both nicols are 
being used, more light is advantageous than when only 
one is in use. As much of the examination of a thin sec- 
tion as possible should be done with the low powers in 
order to save a strain on the eyes. 

The student is particularly cautioned whenever fo- 
cusing with high powers to focus upward and never 
downward. If this rule is followed, no thin sections will 
be broken. Place the eye on a level with the stage, 
and lower the tube slowly until the objective is almost 
in contact with the thin section. Then looking through 
the tube, raise the objective slowly until the portion of 



32 OPTICAL MINERALOGY AND PETROGRAPHY 

the section desired for examination is in focus. If col- 
orless minerals such as quartz are being examined, it 
is well to reduce the amount of the illumination and 
look for bubbles or other inclusions. 

Proper care of the nicols and lenses prolongs their 
life and increases their efficiency. They should not be 
exposed to severe sunlight nor to the heat from a steam 
radiator, lest the cement soften. The lenses should be 
kept free from dust. The objective should never be 
allowed to come in contact with the cover glass. 



METHODS OF MINERAL DETERMINATION 33 



CHAPTER 3. 
General Methods of Mineral Determination. 

The determination of unknown minerals in thin sec- 
tion may be accomplished by the use of one or all of 
the following eight general operations : 

1. Determination of the general physical properties 
of minerals by ordinary light. 

2. Determination of the relative refractive index. 

3. Determination of the relative double refraction or 
birefringence. 

4. Determination of the axial interference figures. 

5. Determination of the dispersion of the optic axes. 

6. Determination of the optical character or the 
character of the double refraction. 

7. Determination of the extinction angle or the rela- 
tion of the crystallographic axes to the axes of ether 
elasticity. 

8. Determination of the presence or absence of pleo- 
chroism. 

General Operation No. 1: Determination of the Gen- 
eral Physical Properties of Minerals by Means of Ordi- 
nary Light. 

The physical properties of minerals referred to in 
this paragraph are: crystal form, cleavage, parting, 
twinning, and color. Ordinary light is light which has 
not been polarized to obtain which both nicols should be 
removed from the microscope. If the lower nicol is 
difficult to remove, the observations are made in plane 



34 OPTICAL MINERALOGY AND PETROGRAPHY 

polarized light, which generally causes no great difference 
in the appearance of the mineral. The intensity of the 
unpolarized light, however, is much greater than that of 
the polarized. 

Minerals examined by ordinary light are of two 
classes: transparent and opaque. The former class is 
examined by transmitted light for crystal form, cleav- 
age, and color ; the latter class by incident light for crys- 
tal form, color, luster, etc. 

CRYSTAL FORM. The determination of crystal form 
is not of great importance in the study of rock sections 
for the reason that individual crystals have not had the 
opportunity for undisturbed development, but have been 
hampered in their growth by interference with neigh- 
boring crystals. In certain porphyries a study of the 
form of the phenocrysts often leads to their identification. 

CLEAVAGE. Pronounced cleavage lines are developed 
in certain minerals in characteristic directions during 
the process of grinding to thin section. The direction 
and perfection of the cleavage cracks is indicative. A 
mineral possessing no cleavage will have irregular 
cracks, as quartz. 

Perfect cleavage is a cleavage in which the lines are 
sharp and extend for considerable distances. Examples : 
mica, fluorite. 

Good or distinct cleavage is cleavage in which the 
cracks are interrupted with offsets, etc. Examples: 
augite, hornblende, orthoclase. 

Poor or indistinct cleavage is very irregular, with 
uneven cracks, though they follow roughly certain direc- 
tions. 

Pinacoidal cleavage, as shown in mica, is well devel- 
oped in one direction only. Prismatic cleavage, as 



METHODS OF MINERAL DETERMINATION 35 

shown by augite and hornblende, usually develops in 
two planes. In certain minerals of the isometric and 
hexagonal systems, such as galena and calcite, three 
good cleavages develop. 

Cleavage angles, of course, depend upon the orien- 
tation of the random section shown in the thin section. 
Where the section is cut at right angles to the cleavage 
planes, the angles are characteristic. Hence, if one is 
using cleavage fragments, he can orient it at will, since 
the flat faces will bear definite relations to the crystal- 
lographic axes. 

PARTING. Parting is a fracture often developed par- 
allel to a certain cleavage direction occurring along 
planes of weakness as may result from shearing or glid- 
ing planes. 

TWINNING. Twinning is important in certain min- 
erals and will be discussed in Chapters 5, 6, and 7. 

COLOR. All colored minerals may be divided into 
two classes : idiochromatic and allochromatic. Idiochro- 
matic minerals are those in which the color is due to 
a property of the mineral itself, namely, its ability to 
absorb light of certain wave-lengths, although the prop- 
erty of absorption may not be the same in every direc- 
tion. Allochromatic minerals are those in which the 
color is due to inclusions, which may or may not be dis- 
tinguished under the microscope. The pigment may 
be either organic or inorganic. Carbon, nitrogen and 
hydrogen have been found in zircon, smoky quartz, ame- 
thyst, fluorite, apatite, calcite, microcline, barite, 
halite and topaz. Free fluorine has been found in fluor- 
ite. Traces of iron are found in brown zircon. The 
pigment may be thickly and evenly distributed or irreg- 
ularly and so sparingly distributed that a thin section 
appears colorless. 



36 OPTICAL MINERALOGY AND PETROGRAPHY 

General Operation No. 2: Determination of the Rel- 
ative Refractive Index. 

Refraction is the change which light undergoes in 
direction in passing between two media which differ in 
density. The index of refraction described under optics 
may often be judged by the appearance of the mineral 
in the liquid in which it is mounted, usually Canada bal- 
sam. This makes a convenient standard with which to 
compare the index of refraction of the unknown mineral. 
Although its index varies slightly during the process 
of mounting and with age, only those few minerals 
whose indices fall within the limits of variation of the 
balsam are affected. Balsam may retain its sticky con- 
sistency and low index for forty years if protected by 
a cover glass. 

If the mineral under examination and the balsam 
have practically the same index of refraction, the min- 
eral will appear smooth and will be visible with difficulty. 
It is then said to have "low relief." If the two have 
quite different indices, the surface of the mineral will 
appear rough and the borders dark. Such a surface, 
because of its resemblance to shagreen, is called a sha- 
green surface. The mineral is said to have "high re- 
lief." This apparently rough surface is due to inequali- 
ties of the surface, each elevation and depression re- 
flecting and refracting the light at a different angle. 
This irregular illumination causes the mineral to appear 
darker in some spots and lighter in others. A mineral 
embedded in Canada balsam will have high relief whether 
its index is lower or higher than the liquid. 

In a rock section where a number of different min- 
erals having different indices of refraction lie in con- 
tact, certain minerals appear to stand out above the oth- 
ers in relief. Minerals with high indices seem to be 
elevated from the plane of the section. This is because 



METHODS OF MINERAL DETERMINATION 37 

the rays of light from the lower surface of different 
minerals appear to come from the points of intersection 
of the refracted rays. 

Since the index of refraction of a mineral is one of 
its most important optical properties, many methods 
have been devised for its identification. 

THE METHOD OF Due DE CHAULNES. By this method 
one may determine the index of the mineral directly 
by focusing a medium- or high-power objective accu- 
rately upon an object, and then inserting between it and 
the objective a transparent plate with parallel sides. The 
image becomes blurred. The tube of the microscope is 
raised until the image is again in focus. The amount 
of change necessary is dependent upon the index of 
refraction of the plate and upon its thickness. 

As a laboratory illustration the student is advised to 
determine the index of refraction of a plate glass or 
cover glass by this method. If the student uses a glass 
whose true thickness has already been determined, the 
index may be obtained by measuring the apparent thick- 
ness and dividing the true thickness by this latter 
amount. If the thickness of the glass is not known, it 
is possible to determine the approximate true thickness 
by focusing on a point or scratch on another plate of 
glass which is to be used for a support. Then place 
upon it the glass plate whose thickness is to be deter- 
mined, and focus on its upper surface. This distance 
is the thickness of the glass plate plus the thickness 
of the air film separating the plate from the support, 
and should be used only in case the thickness of the 
plate is so great that the thickness of the air film be- 
comes negligible. The apparent thickness can now be 
measured and the index calculated as mentioned above. 

A correction for the air film can be made very easily 
and should always be done if the glass plate is thin. 



38 OPTICAL MINERALOGY AND PETROGRAPHY 

Focus on the upper surface of the glass plate and then 
on the lower surface. This distance through which the 
objective moves is the apparent thickness of the plate. 
Now focus on the surface of the support. This gives 
the true thickness of the plate and the air film. Next 
focus on the lower surface of the plate and on the upper 
surface of the support. This gives the true thickness of 
the air film, which can readily be subtracted from the 
thickness of the glass plate and air film. The difference 
is the true thickness of the glass plate. The index can 
now be determined by dividing the true thickness by 
the apparent thickness of the plate. 

IMMERSION METHOD. If a drop of a liquid with an 
index equal to that of the mineral is placed upon a thin 
section of the mineral without a cover glass, the appear- 
ance of roughness which characterizes the mineral in 
air disappears, since there is neither reflection nor re- 
fraction at the contact, and the light passes through 
without deflection. If the mineral is colorless, it prac- 
tically disappears from view. By the use of a series 
of immersion liquids whose indices of refraction are 
known, it is possible to experiment with the unknown 
mineral until a liquid is found whose index of refraction 
by the above test corresponds with the index of refrac- 
tion of the mineral. 

BECKE METHOD. By the Becke method, which in- 
volves the use of total reflection in connection with re- 
fraction, one may determine the relation which the re- 
fractive index of the unknown mineral has to that of 
one which is known and which is in contact with it. 
Bring the focus directly upon the line of separation of 
the two minerals, using a high-power objective in con- 
vergent light. If the condenser is lowered and the an- 
alyzing nicol is removed, it is observed that the field 
becomes slightly darker, and a fine line of white light 



METHODS OF MINERAL DETERMINATION 39 

sharply marks the contact of the two minerals. Upon 
raising the objective very slightly, this thin line of white 
light will be seen to shift from the line of contact of 
the two minerals toward the mineral having the higher 
index. 

This phenomenon is explained as follows: The rays 
of light which enter the minerals perpendicular to their 
surfaces undergo no refraction but pass directly through. 
Those rays which enter the mineral having the lower 
index of refraction reach the plane of contact of the 
two minerals and are all bent toward a normal to .this 
plane passing through the mineral having the higher 
index, because they are passing from a rarer to a denser 
medium. The rays which enter the mineral having the 
higher index must pass from a denser medium to a 
rarer. In such a case it is remembered that all of those 
rays which strike the mineral of lower index at an angle 
greater than the critical angle of the denser mineral, 
are totally reflected and emerge from the upper surface 
of the denser mineral. Only those rays pass into the 
mineral of lower index which strike the plane of contact 
of the two minerals at an angle less than the critical 
angle of the mineral of higher index. 

Upon lowering the objective slightly, the white line 
shifts toward the mineral with the lower index. 

It is advisable to use the Becke test on contacts which 
are nearly or quite vertical. One can easily determine 
a vertical contact by shifting the focus and seeing that 
the boundary remains sharp at all foci, and in the same 
position. The verticality of the contact makes no differ- 
ence with the result, provided the medium having the 
lower index lies above. If it lies below and the incli- 
nation is great enough, the bright line may appear to 
move the wrong way. 

According to this method, differences of .001 be- 



40 OPTICAL MINERALOGY AND PETROGRAPHY 

tween indices are noticeable. It is especially useful in 
determining minerals with low indices, as sodalite, leu- 
cite, or in distinguishing between orthoclase and quartz. 
Since the mean refractive index of quartz is about the 
same as andesine, is higher than orthoclase, albite and 
oligoclase, and less than labradorite, bytownite and anor- 
thite, certain definite inferences may be drawn regard- 
ing these minerals in contact. 

The following scale of refringence (after Winchell) 
will aid the student to estimate the value of the mean 
index of refraction of minerals in thin section by means 
of "reliefs." 

SCALE OF REFRINGENCE. 

Very low refringence. Example, fluorite, n = 1.434. 

Low refringence. Example, quartz, n = 1.547. 

Moderate refringence. Example, hornblende, n = 1.642. 

High refringence. Example, augite, n = 1.715. 

Very high refringence. Example, zircon, n = 1.952. 

The "negative" relief seen in fluorite is caused by the 
total reflection of light striking the lower surface of 
the mineral. . 

The indices of an unknown mineral compared with 
those of any known mineral by the Becke method will 
always give at least one limit, which in connection with 
the visible amount of relief may be sufficient. 

General Operation No. 3: Determination of the Rel- 
ative Double Refraction or Birefringence. 

ISOTROPIC CRYSTALLINE SUBSTANCES. ISOMETRIC 
MINERALS. Between crossed nicols, isometric minerals 
remain dark in thin section during the entire revolution 
of the stage. Such minerals allow the rays to vibrate 
with equal ease in all directions regardless of the direc- 
tion in which the section is cut. They have no inter- 



METHODS OF MINERAL DETERMINATION 41 

ference colors in parallel polarized light nor interference 
figures in convergent light. 

This is due to the fact that, in isotropic substances, 
light is transmitted with equal velocities in all directions ; 
hence the velocity of light transmission is independent 
of the direction of vibration. 

Certain important isometric minerals, as pyrite and 
magnetite, are opaque. These minerals are examined 
and identified by other means than by polarized light. 

ANISOTROPIC SUBSTANCES. Uniaxial minerals, or 
minerals of the tetragonal and hexagonal systems. 

A ray of light emerging from the polarizer (lower 
nicol) is vibrating in one plane only, left and right. 
Upon entering a thin section of a tetragonal or hexag- 
onal mineral cut perpendicular to the vertical crystallo- 
graphic axis c, the light is not disturbed, because about 
this axis vibrations take place with equal ease in all di- 
rections perpendicular to it. In this one direction, light 
is singly refracting. 

The optic axis is that direction in a doubly refracting 
substance in which light is singly refracting. Hence 
in uniaxial minerals the crystallographic axis c coincides 
with the optic axis. 

This ray of light emerging from the mineral is in- 
tercepted by the analyzer so as to produce darkness. The 
student should take the precaution, upon observing a 
mineral which remains dark throughout a complete rev- 
olution of the stage, to determine whether it is an iso- 
metric mineral or an isotropic mineral cut perpendicu- 
lar to an optic axis. 

A ray of light from the polarizer entering the thin 
section of an anisotropic mineral in any other direc- 
tion than perpendicular to the optic axis is doubly re- 
fracted and polarized, the extraordinary and the ordi- 
nary rays advancing in different directions, the former 



42 OPTICAL MINERALOGY AND PETROGRAPHY 

taking the oblique direction. These rays are, of course, 
vibrating perpendicular to each other, and perpendicular 
to their direction of propagation. The extraordinary 
ray vibrates in a plane containing the incident ray and 
optic axis. This plane is called the principal optic section. 

On emerging from the thin section, the extraordinary 
ray is more or less advanced than the ordinary ray. 
Upon reaching the analyzer, each of these rays is again 
resolved into two rays an extraordinary and an ordi- 
nary the two extraordinary rays vibrating in one 
plane, and the two ordinary rays in a plane at right 
angles. Upon reaching the layer of Canada balsam, the 
ordinary rays are totally reflected and absorbed. The 
two extraordinary rays emerge from the analyzer in a 
uniform direction, but not equally advanced, conse- 
quently in different phases. This interference produces 
color. If the rays have a difference of phase of one half 
of a wave-length, or any uneven multiple thereof, dark- 
ness will result. 

COLOR. The kind of color produced depends upon : 

1. The mineral. 

2. The thickness of the section. 

3. The direction in which the section is cut. 

The amount of color depends upon the angle between 
the principal optic section and the principal section of 
either nicol. The color is least when the angle is de- 
grees, and greatest when the angle is 45 degrees, each 
of these conditions occurring four times in one revolu- 
tion of the section. Upon this phenomenon is based 
the determination of the angle of extinction (General Op- 
eration No. 6). 

AXES OF ETHER VIBRATION. The direction in which 
ether vibrates in anisotropic minerals with the greatest 
ease is called the greatest axis of ether vibration. It 



METHODS OF MINERAL DETERMINATION 43 




Fig. 7. Changes of light in passing through 
a petrographical microscope. 



44 OPTICAL MINERALOGY AND PETROGRAPHY 

is denoted by the letter X. The index of refraction of 
light vibrating in this direction is expressed by Wp. 

The direction in which ether vibrates in anisotropic 
minerals with the least ease is called the least axis of 
ether vibration. It is denoted by the letter Z. The in- 
dex of refraction of light vibrating in this direction is 
expressed by n g . 

In uniaxial minerals, one of these axes always coin- 
cides with the vertical crystallographic axis c. The 
other axis is in all directions at right angles to this. 
Either the greater or the lesser axis of ether vibration 
may coincide with the vertical crystallographic axis. 

The value of the maximum double refraction or bire- 
fringence is the difference between Hp and n g . Thus, 
for calcite, n g is 1.658 and n? is 1.486 ; n s n = 0.172, 
which indicates a very strong birefringence. For quartz, 
n g is 1.553 and n p is 1.544 ; n s n p = 0.009, which indi- 
cates weak birefringence, as the retardation of one ray 
over the other in emerging is very slight. 

It will be observed that in certain minerals the in- 
dex of refraction of the extraordinary ray is greater 
than that of the ordinary ray, and in other minerals 
the reverse is true. A further discussion of this fact 
will be taken up under General Operation No. 5. 

NEWTON'S COLOR SCALE. Thin sections of anisotropic 
minerals cut not perpendicular to an optic axis show 
polarization colors between crossed nicols. A careful 
study of these colors is most important for a successful 
determination of unknown minerals. 

The color scale of Newton has been adopted as a 
standard. It consists of a succession of interference 
tints shading into each other. These same tints are 
produced by anisotropic minerals in thin section. About 
forty distinguishable tints in natural light have been 



METHODS OF MINERAL DETERMINATION 



45 



named. The colors are best exhibited by thin sections 
which have a thickness ranging between 0.01 and 0.06. 

Newton's color scale as applied to the principal rock- 
making minerals in sections 0.03 mm thick is given here- 
with. 

NEWTON'S COLOR SCALE. 



Millionth* 
of a 


Interference of Colors 


n-n 


Rock-forming 


mm Re- 


Between X Nicols. 


K 1> 


Minerals. 


tardation 











Black 






30 


Iron gray. 


0.001 


Leucite. 


60 


Lavender gray. 


0.002 


Vesuvianite. 


117 


Lavender gray. 


0.004 


Apatite. 


140 


Grayish blue. 


0.005 


Beryl. 


153 


Grayish blue. 


0.005 


Nephelite, riebeckite. 


180 


Lighter gray. 


0.006 


Stilbite, zoisite. 


218 


Lighter gray. 


0.007 


Orthoclase, microcline, 








kaolin. 


234 


Greenish white. 


0.008 


Oligoclase, albite, labra- 








dorite. 


250 


White. 


0.009 


Corundum. 


267 


Yellowish white. 


0.009 


Gypsum, enstatite. 


281 


Straw yellow. 


0.009 


Quartz, sapphire. 


306 


Light yellow. 


0.010 


Topaz, rhodonite, stauro- 








lite. 


332 


Bright yellow. 


0.011 


Clinochlore, barite. 








Andalusite. 


390 


Orange yellow. 


0.013 


Anorthite, hypersthene. 


433 


Orange yellow. 


0.014 


Wollastonite. 


474 


Orange red. 


0.016 


Cyanite. 


575 


Violet (Sensitive tint 








No. 1) Green, Yel. 


0.019 


Hedenbergite. 


589 


Indigo. 


0.020 


Tourmaline. 


629 


Blue. 


0.021 


Wernerite. 


667 


Sky blue. 


0.022 


Augite. 


688 


Sky blue. 


0.023 


Hornblende. 


713 


Greenish blue. 


0.024 


Diallage. 


747 


Green. 


0.025 


Actinolite. Augite. 


810 


Light green. 


0.027 


Tremolite, arfvedsonite. 


855 


Yellowish green. 


0.029 


Diopside, cancrinite. 



46 



OPTICAL MINERALOGY AND PETROGRAPHY 



Millionths 

of a Interference of Colors 

mm Re- 
tardation 

1079 

1228 



1140 
1260 
1300 
1425 
1495 
1652 

1845 
2170 
2900 
3600 
4600 
5200 
5400 
6100 
7200 
8400 
8600 



Between X Nicols. 


g p 


Dark orange violet. 


0.036 


Violet (Sensitive tint 




No. 2). 


0.037 


Indigo. 


0.038 


Greenish blue. 


0.042 


Sea green. 


0.044 


Greenish yellow. 


0.048 


Rose red. 


0.050 


Violet gray (Sensi- 




tive tint No. 3). 


0.056 


Greenish gray. 


0.062 


Colors very faint. 


0.072 




0.097, 




0.121 





0.155 


Not distinguishable. 


0.172 




0.179 




0.202 




0.239 




0.280 


With shades of red 




and green. 


0.287 



Rock-forming 
Minerals. 



Olivine, lazurite. 



Epidote. 

Muscovite. 

Phlogopite, anhydrite. 

Limonite. 

Talc. 



Zircon. 

Hornblende. 

Cassiterite. 

Titanite. 

Aragonite. 

Oalcite. 

Dolomite. 

Magnesite. 

Siderite. 

Hematite. 

Rutile. 



The lowest colors of the above scale are the colors 
of the first order, which includes all of the colors up to 
the first violet, which marks the limit of the order. The 
colors of the second and third orders are successively 
higher. In the fourth order, the colors begin to ap- 
proach white light, due to an overlapping of the inter- 
ference. The highest color which the mineral is capable 
of producing is usually taken for comparison with the 
colors of the table. In uniaxial minerals, such colors are 
given by sections cut parallel to the optic axis. 

Determination of the Order of Color Produced by In- 
terference. The rank of an interference color may be 



METHODS OF MINERAL DETERMINATION 47 

determined by means of a "quartz wedge." This is a 
quartz plate of varying thickness, which gives the colors 
of the Newton scale from the grayish blue of the first 
order up. The quartz wedge is mounted on a plate 
of gypsum in such a position that the faster ray in the 
gypsum is the slower ray in the quartz. The gypsum 
plate is made of such thickness that its effect is com- 
pletely compensated by that of the wedge at the middle 
of the latter. Between crossed nicols, darkness will re- 
sult at this point. Upon moving the wedge in either 
direction, the colors rise successively from this zero or 
compensating point to colors of the third order. When 
the wedge is superimposed over the thin section of a 
mineral, the colors rise in the scale if moved in one di- 
rection and fall if moved in the other direction. 

Assume that a mineral is placed upon the stage of 
a microscope between crossed nicols and in its position 
of maximum illumination. Insert a quartz wedge in 
the proper slit in the microscope tube above the thin sec- 
tion, and note the change of colors as the wedge is moved 
over the field, the thin edge being inserted first. If the 
colors rise in the scale from yellow to red to violet to 
blue to green and again to yellow, it is an indication that 
the greater axis of ether vibration of the thin section 
and that of the quartz wedge are parallel. Therefore, 
turn the stage 90 degrees to its former position and in- 
sert the wedge again. The order of the change of colors 
will now be reversed. The colors will fall, indicating 
that the lesser axis of ether vibration of the thin section 
is parallel to the greater axis of the wedge. Move the 
wedge over the mineral until the plate becomes dark or 
gray. This is the "compensation point," where the accel- 
eration of one of the rays of the plate corresponds ex- 
actly to the retardation of the same in the wedge. Re- 
move the mineral from the stage. The interference color 



48 OPTICAL MINERALOGY AND PETROGRAPHY 

that the wedge displays is now the same as that orig- 
inally shown by the mineral. Slowly remove the wedge, 
observing carefully the sequence of colors. The number 
of times that any color recurs until the wedge is re- 
moved gives the order of the original interference color 
of the mineral. 

From a birefringence chart it is possible to deter- 
mine not only the order of birefringence of a mineral 
but the thickness of the section, provided some mineral 
contained in the slide is known. Let us take, as an ex- 
ample, granite in which quartz is easily recognized. It 
is fairly safe to assume that, if there are many frag- 
ments of quartz in the field of view, the fragment with 
the highest interference color is cut parallel to the optic 
axis, and its birefringence has a maximum value, 0.009. 
This value on the color chart is marked by a diagonal 
line, which should be followed toward the lower left- 
hand corner to the intersection with the vertical, giv- 
ing the interference color shown in the slide. The 
ordinate at the point of intersection represents the thick- 
ness of the section. Its value is determined by follow- 
ing the horizontal line through the intersection to the 
scale on the left. This reading gives the thickness of 
the section in millimeters. 

Having thus determined the thickness of the sec- 
tion, find again the highest interference color in a frag- 
ment of the mineral which is to be determined. Take 
the intersection of the horizontal line of thickness in 
the chart with this color. The diagonal line passing 
through this point of intersection indicates the birefrin- 
gence of the mineral in question. 

Double Refraction of Biaxial Minerals. Minerals of 
the orthorhombic, monoclinic and triclinic systems. 

Minerals of these three systems have two optic axes, 



METHODS OF MINERAL DETERMINATION 49 

or two directions in which light is singly refracting; 
hence the name biaxial. Sections cut perpendicular to 
these directions remain dark between crossed nicols dur- 
ing a complete revolution of the stage. The optic axes 
of biaxial minerals never coincide in position with any 
of the crystallographic axes as is true in the case of uni- 
axial minerals. In the orthorhombic system they lie in 
the same plane with two of these axes. 

Biaxial minerals contain greatest and least axes of 
ether elasticity, which, as in uniaxial minerals, are 
denoted by X and Z. In addition, there is a mean axis 
of ether elasticity, denoted by Y. 

OPTIC AXIAL PLANE. The plane containing X and Z 
also contains the two optic axes, and is called the optic 
axial plane, or the optic plane. The mean axis of ether 
elasticity, Y, is normal to this plane, and is called the 
optic normal. 

BISECTRICES. The optic axes intersect each other at 
the point of intersection of the optic plane with the other 
planes of symmetry, if any exist, making equal angles 
on opposite sides of the axes X and Z. Therefore, X 
and Z are known as bisectrices. When X bisects the 
acute angle of the optic axes, it is called the acute bi- 
sectrix. The same is true for Z. When they bisect the 
obtuse angle, they are called obtuse bisectrices. 

Orthorhombic minerals have three axes of ether vi- 
bration parallel to the crystallographic axes. The direc- 
tion of X may be the same as a, b or c, the directions 
of Y and Z varying accordingly, but always at right 
angles to each other. 

In monoclinic minerals, one of the axes of ether vi- 
bration, frequently Y, coincides with the crystallographic 
axis b (the axis of symmetry), and the other two are 
in the plane of symmetry, parallel to the clinopinacoid. 

In the triclinic system, the axes of ether vibration 



50 



OPTICAL MINERALOGY AND PETROGRAPHY 



have no fixed relation to the crystallographic axes. 

The discussion of the determination of birefringence 
of uniaxial minerals is applicable to biaxial minerals. 
The interference colors in sections of biaxial minerals 
normal to the optic elements grade downward in the 
following order from highest to lowest: 

1. Optic normal. 

2. Obtuse bisectrix. 

3. Acute bisectrix. 

4. Optic axis. 

The following scale of birefringence (after Win- 
chell) is useful for comparison in the estimation of the 
birefringence of an unknown mineral. 



SCALE OF BIREFRINGENCE. 



1. Very weak birefringence 

2. Weak birefringence 

3. Moderate birefringence 

4. Rather strong birefrin- 

gence 

5. Strong birefringence 

6. Strong birefringence 

7. Very strong birefringence 

8. Extreme birefringence 



0.0035 or less. Example, leucite. 

0.0035-0.0095. Example, orthoclase. 

0.0095-0.0185. Example, hypersthene. 

0.0185-0.0275. Example, augite. 

0.0275-0.0355. Example, diopside. 

0.0355-0.0445. Example, muscovite. 

0.0445-0.0565. Example, aegirite. 

0.0565. Example, titanite. 



METHODS OF MINERAL DETERMINATION . 51 



CHAPTER 4. 
General Methods of Mineral Determination (Continued). 

General Operation No. 4: Determination of the Axial 
Interference Figures. 

Interference figures are obtained by the use of crossed 
nicols in convergent light. A high-power objective must 
be used. When the eyepiece is removed, a small image 
of the interference figure can be seen. By sliding the 
Bertrand lens into the tube of the microscope, a mag- 
nified image of the figure is obtained, in which case the 
ocular is retained. Strong illumination is necessary, 
with the condensing lens close under the thin section. 
Results are best with monochromatic light, but the ef- 
fects are the same with white light except that the rings 
will be variously colored instead of light and dark. 

This operation aids the observer in distinguishing 
between isotropic, uniaxial and biaxial substances, and 
aids in the determination of the relative double refrac- 
tion of minerals. 

Isotropic minerals show no interference figures. 

UNIAXIAL INTERFERENCE FIGURES: 

a. Sections cut perpendicular to the optic axis or 
vertical crystallographic axis show a dark cross with or 
without colored rings. The arms of the cross are parallel 
to the vibration planes of the nicols, and the figure does 
not move with the rotation of the section. 

b. Sections cut oblique to this position show figures 
which move about the center of the field. The center 
of the figure may even be outside of the field, but upon 



52 OPTICAL MINERALOGY AND PETROGRAPHY 

rotation its dark bars may be seen to move across the 
field. These dark bars remain straight and parallel to 
themselves. 




Fi*. 8. Uniaxial figure. 

If the obliquity of the section is too great, the bars 
will show a curvature upon entering the field and upon 
leaving, but they are straight upon crossing the center 
of the field. The curvature shifts upon crossing the cen- 
ter from one side to the other, thereby differing from 
the biaxial figures, in which the bars remain curved in 
the same direction. 

c. If the section becomes so oblique to the optic axis 
as to approach parallelism to it, the black cross appears 
to break up into hyperbolas which are symmetrically 
placed with respect to the optic axes, and then unite to 
form a dark cross again upon completing the rotation of 
the section. 

Sections which are thick and have a strong double re- 
fraction show the cross and rings clearly and sharply 
outlined, many rings being crowded closely together. 
Thin sections with weak double refraction show broad 
crosses and no rings. The observer may thus deduce 
inferences both in regard to the thickness of the section 
and the strength of the double refraction. 

BIAXIAL INTERFERENCE FIGURES: 

a. Sections cut normal to an optic axis show a series 
of concentric colored curves crossed by a single dark 
bar. The bar changes into a hyperbola and back into a 



METHODS OF MINERAL DETERMINATION 



53 



bar. Sometimes the curves are not observable. The bar 
when straight shows the direction of the optic plane with 
which it is parallel. 










m 





b. Sections cut normal to the acute bisectrix in which 
the angle between the optic axes is not too great will 
show both optic axes in the interference figure, the bisec- 
trix being in the center of the field between them. In 



54 



OPTICAL MINERALOGY AND PETROGRAPHY 



one case a dark bar appears in the center of the field, its 
arms varying in size. That line which passes through the 
optic axes is narrower than the one passing between 




Fig. 10. Optic axis 
interference figure. 

them. Its extremities widen out on the edge of the field. 
The intersection of the two bars marks the bisectrix. 
The trace of the optic plane is the line passing through 
the loci of the optic axes and the bisectrix. On rotating 
the section, the dark bars separate into two hyperbolas, 
the summits receding from each other toward the edge 
of the field, and beyond it if the optic axes are not in 
view. They bend through the colored curves surround- 
ing the optic axes, and unite again as a straight bar when 




Fig. 11. Bisectrix interference 
figure. 




Fig. 12. Bisectrix interference 
figure at 45. 



the plane of the optic axes coincides with the vibration 
plane of the nicol. The most distant positions of the 
hyperbolic summits are, therefore, after a revolution of 
45 degrees. 



METHODS OF MINERAL DETERMINATION 



55 



An excellent illustration of the biaxial interference 
figure may be obtained very simply by placing between 
crossed nicols in convergent light a thin sheet of musco- 




) 






CD 










- 




vite mica. Since the optic angle is small, the loci of both 
optic axes will be seen in the field. Since the center of 
the small ellipses and the black hyperbolas mark the loci 



56 OPTICAL MINERALOGY AND PETROGRAPHY 

of the optic axes, they indicate approximately the optic 
angle. 

The uniaxial or biaxial character of a mineral sec- 
tion which shows only an indistinct bar may be deter- 
mined as follows (La Croix) : 

A bar of a uniaxial interference figure moves in the 
same direction as the rotating stage, and always remains 
straight, while the biaxial bar moves in the opposite 
direction to that of the stage, and becomes curved. 

General Operation No. 5: Determination of the Dis- 
persion of the Optic Axes. 

The colors of the interference figures in convergent 
light are caused by the difference of phase of different 
rays brought together by the analyzer so as to inter- 
fere. The phenomenon of relative position of the 
red and violet rays is caused, by the dispersion of the optic 
axes. When the red ray has the greater optic angle it is 
expressed by R > V ; when the violet ray has the greater 
optic angle, it is expressed by R < V. 

When white light is used, the colors on the convex 
side of the hyperbola (which is the side toward the acute 
bisectrix) are edged with red if the dispersion of red is 
greater than that of violet, and edged with violet if the 
reverse is true. 

Labradorite, muscovite, orthoclase, and anorthite 
have a dispersion formula R > V. Albite and oligoclase 
have a dispersion formula V > R. 

General Operation No. 6: Determination of the Op- 
tical Character or the Character of the Double Refraction 

(after Winchell) : 

OPTICAL CHARACTER OF UNIAXIAL MINERALS. For 
light traveling perpendicular to the optic or vertical crys- 
tallographic axis, the vibrations of the ordinary ray are 
transverse to that axis and those of the extraordinary 



METHODS OF MINERAL DETERMINATION 



57 



ray are parallel to it; o may be greater or less than e, 
as the vertical axis may be greater or less than the hori- 
zontal crystallographic axes. 

If the vertical axis is the direction of the greater 
axis of ether vibration (X) , the mineral is optically nega- 
tive. The extraordinary ray is less refracted than the 
ordinary ray, and advances with greater velocity. This 
is expressed as o > e. Optically positive minerals are 
those in which o < e. The greater the velocity, the less 
the refraction, and the smaller the index of refraction. 

To determine the sign of an unknown mineral, one 
must be able to compare the relative velocities of the 
ordinary ray (vibrating perpendicular to the primary 



l 




f 








Fig. 16. Quarter-undulation mica plate. 

axis) and the extraordinary ray (vibrating parallel to 
the primary axis) within the thin section, with the 
known velocities of another mineral. This may be ac- 
complished by the following methods : 

A. With the quarter-undulation mica plate in parallel 
polarized light. 

The quarter-undulation mica plate consists of a cleav- 
age leaf of muscovite, the thickness of which is just 
sufficient to produce a retardation of a quarter of a wave- 
length of light, or about two of the larger vertical divi- 
sions of the color chart. It is mounted between .two glass 
plates. An arrow on the glass usually indicates the direc- 
tion of the lesser axis of ether vibration (Z). 

When the coinciding axes of the mica plate and the 



58 OPTICAL MINERALOGY AND PETROGRAPHY 

mineral are the same (Z), the double refraction is in- 
creased in proportion to the resultant thickness of the two 
plates. The color of the section rises through two of the 
vertical divisions of the color chart. The color falls cor- 
respondingly if the axes are not the same. 

B. With the quartz-sensitive tint in polarized light. 

The quartz-sensitive tint is a plate of quartz cut 
parallel to its vertical or lesser axis of ether vibration, 
and is of such a thickness as to give the first violet color 
of Newton's scale. It is mounted between glass plates. 
The direction of the vertical crystallographic axis, as well 

In parallel polarized light. 





r~o\ 


> 


QUARTZ PLATE \^.2\ 
Sensitive tint % ^"-N 

>W \lL \ 


vjE < CO / 



Color falls : Negative. 
Fig:. 17. Use of the quartz-sensitive tint. 

as the optic axis, corresponds with the direction of Z, 
and is usually indicated by an arrow on the glass. 

The position of the axes of ether vibration in the un- 
known mineral is first determined. This is done by deter- 
mining on rotation between crossed nicols, the positions 
at which extinction takes place. The section is placed 
45 degrees to this position. The brightest interference 
color is thus produced. Place the quartz-sensitive tint 
over the section in such a position that the direction of Z 
is 45 degrees with the principal sections of the nicols. 
If by this superposition a color is produced which is 
higher in the scale than the sensitive tint of the quartz 



METHODS OF MINERAL DETERMINATION 59 

plate, the axis Z of the quartz plate is parallel to the axis 
Z of the thin section. If the resultant color is lower, 
the axis of ether vibration of the mineral is X, in conse- 
quence of a lessening of the retardation. 

Uniaxial minerals are positive when Z coincides with 
the optic axis, and negative when X does. Since the 
optic axis coincides with the vertical crystallographic 
axis, it is necessary when using this method to be able by 
the crystal outline to determine the direction of the 
vertical axis. This method is, therefore, practicable only 
when the section is approximately parallel with the ver- 
tical axis and the crystal outline is distinct. 

C. With the quartz wedge in parallel polarized light. 
This method is the same as method B. 




Fie. I*. Disturbance of the interference figure of a uniaxial 
crystal by the quarter-undulation mica plate. 

D. With the quarter-undulation mica plate in conver- 
gent light. 

By inserting the plate with its Z axis 45 degrees with 
the cross hairs, the dark cross of the interference figure 
is destroyed and two dark spots are brought prominently 
into view. If rings are seen, they will appear disjointed 
at the lines dividing the quadrants, and they will appear 
expanded in those quadrants occupied by the dark spots. 

The mineral is optically positive if a line joining the 
two dark spots is perpendicular to the axis of the mica 
plate. The mineral is negative if the line uniting the 



60 OPTICAL MINERALOGY AND PETROGRAPHY 

dark spots is parallel with the direction of the arrow on 
the mica plate. 

The positive and negative character of the mineral 
becomes a simple operation if it is borne in mind that 
the line joining the dark spots makes a positive sign and 
a negative sign respectively with the axis of the mica 
plate, thereby indicating directly the sign of the mineral. 

E. With the quartz-sensitive tint in convergent light. 

Upon inserting the sensitive tint plate, two opposite 
quadrants will appear yellow and the other set will ap- 
pear blue. In determining the sign of the mineral, the 
yellow quadrants may be considered equivalent to the 
dark spots. 

When a section is cut parallel with the optic axis, 
the interference figure is not a black cross but may re- 
semble a biaxial interference figure. The observer wishes 
to determine the direction of the optic axis. He may 
determine this by observing in which quadrants the 
hyperbolas always leave the field. These will be the quad- 
rants containing the optic axis. Moreover, the inter- 
ference colors in these quadrants are lower than for 
corresponding points in the other quadrants. After 
once determining the direction of the optic axis, Z can be 
determined by any one of the first three methods for 
parallel polarized light. 

OPTICAL CHARACTER OF BIAXIAL MINERALS. If the 
greatest axis of ether vibration bisects the acute bisec- 
trix, the mineral is negative. If Z bisects the acute bisec- 
trix, the mineral is positive. Therefore, a determination 
of the optic sign of a biaxial mineral demands a distinc- 
tion of the acute from the obtuse bisectrix and a distinc- 
tion of X and Z. 

Distinction between the Acute and Obtuse Bisectrices. 
The thin section is cut perpendicular to the acute bisec- 



METHODS OF MINERAL DETERMINATION 61 

trix if the optic angle is so small that the loci of the 
optic axes or of one optic axis and the bisectrix remain 
in the field during a rotation of the stage. Otherwise 
it is necessary to find sections cut perpendicular to both 
X and Z and compare them. 

1. The section perpendicular to the acute bisectrix 
shows a lower interference color than the section cut 
perpendicular to the obtuse bisectrix. 

2. The angle of rotation between the position of the 
black cross and the position when the summits of the 
hyperbolas are tangent to the edge of the field can be 
measured. This angle is larger in a section perpendicu- 
lar to an acute bisectrix than in one perpendicular to 
the obtuse bisectrix. If the angle is more than 30 or 35 
degrees, it is safe to assume that the section is perpen- 
dicular to an acute bisectrix. If the angle is less than 15 
or 20 degrees, it is perpendicular to an obtuse bisectrix. 

Distinction between X and Z. This involves a com- 
parison of the velocities of the light ray in the direction 
of the axis to be determined with that in the direction of 
a known velocity in another mineral. Relative retarda- 
tion is indicated by the relative positions of the colors on 
Newton's scale. The following methods are available: 

A. With the quartz-sensitive tint in parallel polar- 
ized light. 

The section examined must be parallel to the optic 
plane, that is, it must contain the axes Z and X. Z of the 
quartz plate lies in the direction of the arrow, and X at 
right angles to this direction. Superpose the quartz plate 
over the mineral. If the resultant color is higher than 
the sensitive tint of the quartz plate, the Z axes of the 
quartz plate and of the mineral are coincident. If the 
resultant color is lower, the Z axis of the quartz plate is 
coincident with the X axis of the mineral. This enables 



62 OPTICAL MINERALOGY AND PETROGRAPHY 

the observer to determine the position on the two axes. 

It is now necessary to view the interference figure 
in convergent light in order to determine in which quad- 
rants the optic plane lies. If Z is found to lie in the acute 
optic angle, it bisects the acute bisectrix, and the mineral 
is positive. 

If only two hyperbolas are observed, they are in the 
quadrants containing the acute bisectrix. If the optic 
angle is large, hyperbolas may be visible in all four quad- 
rants, but the hyperbolas leave the field more slowly in 
the quadrants containing the acute bisectrix. 

The quarter-undulation mica plate may be used in 
the same manner as with uniaxial minerals to determine 
the directions of X and Z since the section is cut parallel 
to the plane containing X and Z. 

B. With the quartz wedge in convergent light. 

Obtain an interference figure from a section as nearly 
normal to the acute bisectrix as possible, and rotate the 
stage until the optic plane makes a 45-degree angle with 
the vibration planes of the nicols. 

Insert the quartz wedge, with the thin edge advanced, 
in such a position that the Z axis coincides in direction 
with a line passing through the optic axes of the figure. 

The optical character of the mineral is positive when 
the ellipses surrounding the loci of the optic axes appear 
to widen out, and move from the loci of the optic axes 
toward the center of the interference figure and finally 
open into the outer colored margins surrounding the 
whole figure. The optical character of the mineral is 
negative if the movement of the colors is reversed from 
the center of the figure toward the axial spots. 

With certain interference figures the following simple 
rule will apply: If the dark spots approach each other, 
the mineral is negative. If they appear to retreat from 
each other, the mineral is positive. 



METHODS OF MINERAL DETERMINATION 63 

C. With the quarter-undulation mica plate in con- 
vergent light. 

This plate is perpendicular to the negative bisectrix 
X, and contains Z and Y. The direction of Z coincides 
with the trace of the plane of the optic axes, since the 
axial plane always contains X and Z. 

For sections perpendicular to an acute bisectrix. 
When the mica plate is superposed in the usual way, there 
is an apparent lengthening of the figure in the direction 
of the Z axis of the mica plate and an apparent short- 
ening in this direction for positive minerals. Winchell 
suggests that this observation be made with the optic 
plane parallel with one nicol. The dark spots will now 
appear in the quadrants through which the arrow passes, 
the line connecting them forming an angle less than 45 
degrees with the arrow, or an approximate minus sign 
indicating a negative mineral. The reverse takes place 
with a positive mineral. There is a shortening in the 
direction of the arrow, and the line connecting the dark 
spots forms an angle greater than 45 degrees with the 
arrow, making an approximate plus sign. 

Iddings suggests the following method for sections 
perpendicular to an optic axis : Place the section with its 
optic plane 45 degrees with the nicols. The hyperbola is 
convex toward the acute bisectrix. Insert the mica plate 
with the Z axis parallel with the optic plane of the min- 
eral. The hyperbola moves toward the obtuse bisectrix 
when the mineral is negative, and toward the acute bisec- 
trix when the mineral is positive. For minerals of weak 
birefringence, as the feldspars, this method is excellent. 

For minerals of strong birefringence the following 
rule may be applied: The mineral is positive when the 
black dot appears on the convex side of the hyperbola 
upon insertion of the mica plate with its Z axis parallel 
with the optic plane of the mineral. 



64 OPTICAL MINERALOGY AND PETROGRAPHY 

SUMMARY OF THE OPTICAL SIGN FOR UNIAXIAL 
MINERALS. When the E ray is less refracted than the O 
ray and advances with greater velocity, the mineral is 
negative, as in calcite. In this case, X coincides with the 
optic or vertical axis. The index of refraction for the E 
ray vibrating in this direction is the lesser one, rip. 

When the ray is less refracted than the E ray and 
advances with the greater velocity, the mineral is posi- 
tive, as in quartz. In this case, Z coincides with the verti- 
cal or optic axis. The index of refraction of the E ray 
which is vibrating in this direction is the greater one, n g . 

FOR BIAXIAL MINERALS. When X is the acute bisec- 
trix, the mineral is negative, as in muscovite. 

When Z is the acute bisectrix, the mineral is posi- 
tive, as in augite. 

General Operation No. 7: Determination of the Ex- 
tinction Angle, or the Relation of the Grystallographic 
Axes to the Axes of Ether Vibration. 

This operation is performed between crossed nicols 
with parallel polarized light. 

It will be remembered that the intensity of color de- 
pends upon the angle between the principal optic sec- 
tion of the mineral and the principal section of either 
nicol, the color being greatest at 45 degrees and least 
at degrees. Thus when the section is in such a posi- 
tion that its directions of elasticity are parallel to the 
vibration planes of the nicols, no light can pass through 
the analyzer, and the section is dark. This phenomenon 
is called extinction. 

Extinction is the most common phenomenon for dis- 
tinguishing isotropic minerals from anisotropic. In 
the isometric system all minerals are completely dark 
during a rotation of the stage. It is likewise of great 



METHODS OF MINERAL DETERMINATION 65 

importance in distinguishing between minerals of the 
three biaxial systems. 

Extinction is said to be parallel when the directions 
of the axes of ether elasticity are parallel to any crys- 
tallographic directions, which may be determined by 
cleavage, crystal boundaries, or twinning lines. 

Parallel extinction is shown by all sections of tet- 
ragonal, hexagonal, orthorhombic minerals, and in the 
monoclinic minerals in sections parallel to the b axis 
or orthozone. Oblique extinction is shown in all other 
sections of the monoclinic minerals and all sections of 
triclinic minerals. In the triclinic system there is no 
coincidence between the axes of elasticity and the crys- 
tallographic axes. 

The angle between an axis of elasticity in the sec- 
tion and some known crystallographic direction is called 
the extinction angle. It is measured as follows : Find 
the positions of the axes of elasticity in the section when 
extinction takes place. Note the reading on the vernier 
of the stage. Rotate the vernier until the known crys- 
tallographic direction is brought into a parallel posi- 
tion with the same cross hair which was previously used. 
A more distinct view of the field may be obtained by 
removing the upper nicol. The difference between these 
two readings is the extinction angle. 

In monoclinic minerals the maximum value of the 
extinction angle, which is the only angle of real value 
in differentiating the mineral, is obtained from a section 
parallel to the clinopinacoid. Results accurate enough 
for all practical purposes may be obtained by measuring 
the angle of all sections of the mineral and taking the 
maximum value obtained. 

Amphiboles and pyroxenes are easily distinguished in 
this manner. 

Extinction which passes over the section like a dark 



66 OPTICAL MINERALOGY AND PETROGRAPHY 

wave or shadow is called undulatory extinction. It in- 
dicates that the mineral has been subjected to mechanical 
forces, producing a change in the position of the axes 
of elasticity in different parts of the mineral. 

It is difficult for the eye to distinguish small varia- 
tions in the intensity of light. By the use of the quartz- 
sensitive tint, extinction is determined quite accurately 
by a distinction of difference of color to which the eye 
is more susceptible. A thin quartz plate is cut parallel 
to the axis of elasticity, having such a thickness that 
it shows the violet color of Newton's scale. Insert this 
plate in such a position that its axis is 45 degrees to 
the cross hairs. The field of the microscope is violet. 
By placing the unknown mineral on the stage so as not 
to occupy the entire field, it will be seen that the color 
of the mineral is not the same as the violet color of 
the unoccupied field. Rotate the stage until the color 
of the mineral is the violet color of the quartz plate. 
The mineral is now at extinction. This phenomenon is 
due to the fact that the axes of elasticity of the nicols 
and of the mineral are in the same position and producing 
no interference. 

General Operation No. 8: Determination of the Pres- 
ence or Absence of Pleochroism. 

Pleochroism is a property possessed by all aniso- 
tropic minerals of absorbing certain colored rays in cer- 
tain crystallographic directions, thereby showing differ- 
ent colors in different directions by transmitted light. It 
is observed by polarized or parallel transmitted light. 

The axes of absorption coincide generally with the 
axes of elasticity, therefore with the crystallographic 
axes in the tetragonal, hexagonal, orthorhombic, and the 
b axis of the monoclinic systems. 

Sections perpendicular to the optic axis can not show 



METHODS OF MINERAL DETERMINATION 67 

differences in color since in this direction the absorp- 
tion must be equal in all directions. 

Uniaxial minerals are said to be dichroic, showing 
two different colors, produced by the rays which vibrate 
parallel to the direction of the vertical axis and parallel 
to the plane of the basal axes. 

Biaxial minerals are said to be trichroic, as there 
are theoretically three differences in color, corresponding 
to the directions of the three axes of elasticity. Ple- 
ochroism exists practically only'in colored minerals. 

Pleochroism may be tested as follows: If a mineral 
is pleochroic, a change in color will be observed upon 
rotating the stage. This may appear as an actual change 
in color or as a change in shade of the same color. In 
case it is almost indistinguishable, it is best to make 
the test with the condensing lens in position immediately 
under the section. 

An absorption formula is an expression of these dif- 
ferent amounts of absorption in any mineral. Thus 
a > b indicates that absorption is greater when the ether 
vibration of the polarized ray is parallel to the crystal- 
lographic axis a than when parallel to b. 

A pleochroic formula expresses the colors that a min- 
eral presents in polarized light vibrating parallel to each 
of its axes of ether vibration. 

For magnesium tourmaline the pleochroic formula is 
Z = Dark yellowish brown. 
X = Pale yellow. 

In general, amphiboles show pleochroism, and pyrox- 
enes do not. 



68 OPTICAL MINERALOGY AND PETROGRAPHY 



CHAPTER 5. 
Description of Important Rock-making Minerals. 

INTRODUCTION. 

The present-day classification of minerals is pri- 
marily a chemical one, as minerals are arranged accord- 
ing to the acid radical. In any chemical division, how- 
ever, minerals of similar chemical composition, if related 
crystallographically, are placed in the same group, as 
for instance the six members of the calcite group. 

About a thousand kinds of minerals are known, of 
which most are rare or found only in a few localities. 

Rogers has compiled the following information, which 
is of interest in a discussion of the derivation of min- 
eral names : 

The following minerals were named in honor of prom- 
inent scientists : biotite (Biot, French physicist), brucite 
(Bruce, an early American mineralogist) , dolomite (Dol- 
omieu, French geologist), goethite (Goethe, the German 
poet), millerite (Miller, English crystallographer) , 
scheelite (Scheele, Swedish chemist), smithsonite 
(Smithson, founder of the Smithsonian Institution), wol- 
lastonite (Wollaston, English chemist) . 

The following minerals were named from prominent 
geographical localities: andalusite (Andalusia, a prov- 
ince of Spain), aragonite (Aragon, ancient kingdom in 
Spain), anglesite (Anglesea, in Wales), bauxite (Beaux, 
in France), ilmenite (Ilmen mountains, in the Urals), 



DESCRIPTION OF ROCK-MAKING MINERALS 69 

labradorite (Labrador), muscovite (Moscow, in Russia), 
strontianite (Strontian, in Scotland). 

The following minerals were derived from the Latin 
and Greek names for colors: albite (white), azurite 
(blue), cyanite (blue), celestite (sky-blue), chlorite 
(green), erythrite (red), hematite (blood), leucite 
(white), rhodonite (rose-red), rutile (reddish). 

The following minerals were named directly from 
their chemical composition: argentite, arsenopyrite, 
barite, calcite, chromite, cobaltite, cuprite, magnesite, 
molybdenite, sodalite, stannite, zincite. 

At one time there existed a binomial nomenclature 
for minerals, as exist at the present time for animals 
and plants. Thus, barite was known as Baralus ponder- 
osus, and celestite was known as Baralus prismaticus. 

The following minerals are discussed according to the 
crystallographic system in which they occur, isotropic 
minerals being considered first. 

ISOTROPIC MINERALS. 

AMORPHOUS. 

OPAL. 
. . Composition: SiO,. nH 2 0. 

Criteria for determination in thin section : 

Form: No crystal form, but sometimes concretion- 
ary, banded or with spherulitic structure. 

Optical Properties: n = 1.45. Relief so low that the 
mineral may be mistaken for a hole in the section filled 
with balsam. Feeble negative double refraction at times. 
Colorless patches or veins. Fragments are dark and 
irregular between crossed nicols. 

Occurrence: As a secondary mineral in cavities and 
seams in igneous rocks; as sinter around hot springs 
and geysers (Yellowstone Park) ; as a constituent of 



70 OPTICAL MINERALOGY AND PETROGRAPHY 

diatomaceous earth. Diatoms and radiolaria secrete 
casts of opal silica. 

Uses: As gems. Precious opals are found in New 
South Wales and in Hungary. Fire opal is found in 
Mexico. 

ISOMETRIC 
PYRITE. 

Composition : FeS 2 . 

Criteria for determination in thin section : 

Form: Cubes, pentagonal dodecahedrons or combi- 
nations of these. Sometimes in irregular grains. 

Optical Properties: Opaque. In reflected light, pale 
brass-yellow color with strong metallic luster. 

Alteration : Alters readily to limonite by oxidation 
and hydration. 

Occurrence. As a vein mineral associated with other 
sulphides. As an original and secondary mineral in 
igneous and sedimentary rocks. 

Uses: Used in the manufacture of sulphuric acid. 
In association with chalcopyrite as a low grade copper 
ore. It is often gold-bearing. 

PYRRHOTITE. 

Composition: Fe 6 S 7 to Fe^S^. 

Criteria for determination in thin section : 

Form: Practically always in irregular masses and 
not in crystals. Cleavage usually not visible microscop- 
ically. 

Optical Properties. Opaque. Color between bronze 
yellow and copper red. Luster metallic. 

Distinctions : Distinguished from pyrite by its usual 
association in irregular masses and by its bronze yellow 
color in incident light. 

Occurrence: In basic igneous rocks; as a vein min- 
eral ; in crystalline limestones. 



DESCRIPTION OF ROCK-MAKING MINERALS 71 

Uses : The nickeliferous pyrrhotite of Sudbury, 
Ontario, is an important ore of nickel. 

MAGNETITE. 

Composition : Fe ;! 4 . 

Criteria for determination in thin section: 

Form : Octahedrons and dodecahedrons. Also gran- 
ular. Cleavage indistinct. Twinning common after 0. 

Optical Properties: Opaque. Bluish black by 
reflected light, with a strong metallic luster. Index of 
refraction high. 

Alteration : Alters to hematite, limonite and siderite. 

Occurrence: A common and widely distributed 
accessory mineral of igneous rocks; magmatic segrega- 
tion in ore deposits, as in Scandinavia ; as a contact min- 
eral between limestones and igneous rocks; in lenses, in 
gneisses and schists. 

Uses: Important ore of iron, especially in New 
York, New Jersey and Pennsylvania. 

SPINEL. 

Composition : MgAl,0 4 . 

Criteria for determination in thin section : 

Form : In grains or octahedral crystals, never 
decomposed in rocks. 

Optical Properties: Strictly isotropic. Index of 
refraction high. Color usually the lighter shades of red, 
blue-green, yellow, brown. The most common colors are 
green (in pleonaste, iron-bearing) and coffee-brown (ir. 
picotite, chrome-bearing) . 

Differentiation: Distinguished from garnets by its 
octahedral form, by the more common green color, by 
its undecomposed condition and by its slightly lower 
relief. 

Occurrence: As a contact mineral, in crystalline 



72 OPTICAL MINERALOGY AND PETROGRAPHY 

limestones and schists. As an accessory mineral, in igne- 
ous rocks. In the gem-bearing gravels of Ceylon. 
Uses : Ruby-spinel is used as a gem. 

GARNET. 

Composition: R",R'", (Si0 4 ) 3 R" is Ca, Mg, Fe or 
Mn. R'" is Al, or Fe. 

Criteria for determination in thin sections : 

Form: Irregular grains or in simple crystals as 
dodecahedrons. Zonal structure frequent. No cleavage. 
Irregular fracture. 

Optical Properties: Normally isotropic, sometimes 
showing anomalous double refraction, due possibly to 
internal strain. 

Colorless or nearly so, to yellowish or reddish. 

Index of refraction : n = 1.746 to 1.814. 
Relief high and surface rough. 

Alteration : Usually fresh. May be found altered to 
chlorite. 

Differentiation: From spinel, see under the latter. 

Occurrence: In schists and gneisses, granites, peg- 
matites, peridotites, nepheline and leucite-bearing lavas, 
in crystalline limestones developed at the contact, in 
sands. 

Uses: As an abrasive, particularly for finishing 
woodwork and leather. Also as a semiprecious gem. 

LEUCITE. 

Composition : KAlSi 2 O (i . 
Criteria for determination in thin section : 
Form: Grains, or well defined, embedded crystals 
very near the trapezohedron or tetragonal trisoctahe- 
dron. Cross sections round or eight-sided. Vary greatly 
in size. Fine striations due to twinning common. No 
cleavage, though fracture may be noticed. 



DESCRIPTION OF ROCK-MAKING MINERALS 73 

Optical Properties: 

Colorless. Refringence low. Relief absent. Sur- 
face smooth, n = 1.50. 

Birefringence weak but distinct. Colors of first 

order (0.001). 

Inclusions : Symmetrically or zonally arranged, con- 
sisting of the older secretions associated with leucite, 
magnetite, apatite, augite. 

Alteration: Alters readily to zeolites, or mixtures 
of albite and sericite, or orthoclase and sericite. 

Occurrence : Rare in the United States, but common 
in Italy in basic lavas, substituting the feldspars. 

Differentiation : Distinguished from all minerals 
except analcite and sodalite by its low refringence, crys- 
tal form and twinning, very weak birefringence. From 
leucite and sodalite, by its higher refringence. 

SODALITE. 

Composition: 3 NaAlSiO 4 . NaCl. 

Criteria for determination in thin section : 

Form: In concentric nodules. Usually in dissem- 
inated or massive form without crystal faces. Crystals 
if present are dodecahedrons. Fracture conchoidal. 
Cleavage dodecahedral, generally invisible in thin 
section. 

Optical Properties. Isotropic. May be weakly bire- 
fringent around inclusions. 

Relief absent. Surface smooth, n = 1.485. 
Color : Colorless, pink, yellow, blue. 

Alteration: Common to fibrous mass of zeolites or 
to aggregates of micaceous minerals, often accompanied 
by the formation of limonite and calcite. 

Occurrence: In eruptive rocks rich in soda, such as 
nepheline syenites. 

Differentiation: From nephelite by its isotropic 



74 OPTICAL MINERALOGY AND PETROGRAPHY 

character. From other isotropic minerals, by a very low 
refractive index. 

FLUORITE. 

Composition : CaF 2 . 

Criteria for determination in thin section : 

Form: Crystals cubical, octahedral and dodecahe- 

dral. Cleavage, perfect octahedral, appearing often in 

section as triangular cracks. 

Optical Properties: Isotropic. 

n 1.434. On account of the low index of refrac- 
tion the negative relief is marked. 

Abnormal birefringence may show, due to internal 
tension. 

Color is due to inclusions of hydrocarbons. Some 
crystals appear green by transmitted light and blue 
by reflected light. Color not uniformly distributed. 
Occurrence: As a very common vein mineral 

together with calcite, barite, sphalerite, and galena. In 

limestones. Kentucky and Illinois are chief sources. 
Uses : As a flux in iron-smelting and foundry work. 

Also for the manufacture of hydrofluoric acid and 

enamels. 



DESCRIPTION OF ROCK-MAKING MINERALS 75 



CHAPTER 6. 
Description of Minerals (Continued). 

ANISOTROPIC MINERALS. UNIAXIAL. 
TETRAGONAL. 

RUTILE. 

Composition : Ti0 2 . 

Criteria for determination in thin section : 
Form: Embedded grains, acicular inclusions, mas- 
sive or in crystals, which are sharp, elongated and pris- 
matic, or in net-shaped groups. Twinning lamellae com- 
mon in basal sections. Prismatic cleavage not observed. 
Elongation parallel to c. 

Optical Properties: Uniaxial and positive. 

Refringence very high. Relief high and surface 
rough, n = 2.903 and 2.616. 

Birefringence extreme. Interference colors very 
high, hence may not be noticed when mineral is 
strongly colored (0.287). 

Extinction parallel to prisms. 
Color : Red, brownish red to black. 
Pleochroism usually not noticeable. X is yellow- 
ish, Z is brownish yellow to yellowish green. 
Alteration : Quite stable. May alter to ilmenite. 
Occurrence: More widely distributed as a micro- 
scopic mineral than as one of megascopic size. Occurs in 
igneous and metamorphic rocks and in veins. As a sec- 
ondary mineral in clays. Virginia is source. 



76 OPTICAL MINERALOGY AND PETROGRAPHY 

Uses: As a source of ferro-titanium and as a col- 
oring matter for porcelain. 

ZIRCON. 

Composition : ZrSiO 4 . 
Criteria for determination in thin section : 
Form: "Small, short prismatic crystals usually elon- 
gated parallel to c. Always crystallized. 

Optical Properties: Uniaxial and positive. 

Refringence very high and surface rough, n = 
1.983 and 1.93. 

Birefringence very strong (0.053). Interference 
colors of fourth order, minute crystals showing bril- 
liant colors. 

Color : Colorless to pale gray or brown. 
Pleochroism usually not noticeable, but when 
observed little absorption takes place parallel to c. 
Extinction: Parallel to c. 

Interference figure in basal section shows several 
rings in addition to dark cross. 
Alterations : Rare. 

Differentiations: From apatite, by much higher 
relief and stronger double refraction. From cassiterite, 
by much weaker double refraction, and by mode of occur- 
rence. 

Occurrence: As an accessory mineral of igneous 
rocks, especially the more acid varieties. In sands and 
gravels. 

WERNERITE (SCAPOLITE GROUP). 

Composition: mCa 4 Al 6 O 2ri . + nNa 4 Al 3 Si 9 O, 4 Cl. 

Criteria for determination in thin section: 

Form: Crystals rough, coarse and large, in cleav- 
able, columnar and massive forms. Cleavage distinct, 
parallel to square prism. Elongation parallel to a. 

Optical Properties : Uniaxial and negative. 



DESCRIPTION OF ROCK-MAKING MINERALS 77 

Refringence considerable, n = 1.583 and 1.543. 
Relief not marked and about the same as quartz. 

Birefringence rather strong (.03 to .018). Inter- 
ference colors of the second order more brilliant than 
those of most of the colored minerals. 

Interference figures distinctly uniaxial. 

Extinction parallel in longitudinal sections. 

Colorless. 

Alteration : Alters to kaolinite, muscovite, etc. 
Differentiation: From feldspars, by absence of 
twinning. 

From quartz, by cleavage, higher order of inter- 
ference colors and optical character. Quartz is 
positive. 

From apatite, by lower index of refraction, cleav- 
age and higher order interference colors. 
Occurrence: Found in gneisses, crystalline schists, 
and limestones. 

HEXAGONAL. 

HEMATITE. 
Composition : Fe 2 0,. 
Criteria for determination in thin section: 
Form: Irregular scales, minute grains and earthy. 
No cleavage. 

Optical Properties : Uniaxial and negative. 

Refringence very high, n 3.042 and 2.797. 

Birefringence very strong (0.245). 

Opaque. By reflected light, black with tinge of red 
and a metallic luster, or red without luster. 

Pleochroism absent, or slight. X is yellowish red 
and Z is brownish red. 

Alteration : Common by hydration to limonite. 
Occurrence: Very widely disseminated. As micro- 
scopic inclusions and as a common alteration product in 



78 OPTICAL MINERALOGY AND PETROGRAPHY 

all rocks. As a commercial iron ore from the Lake 
Superior district. 

ILMENITE. 

Composition : FeTiO,,. 

Criteria for determination in thin section : 

Form: Irregular masses, without crystallographic 
outline, or rhombohedral crystals. 

Optical Properties : 

Opaque. Rarely translucent, and dark brown in 

very thin sections. Sometimes brownish in reflected 

light, with metallic luster. 

Alteration: To leucoxene, which is believed to be a 
variety of titanite. This alteration often develops along 
definite rhombohedral directions. 

Differentiation: From magnetite, by occurring in 
irregular masses and by a whitish strongly refracting 
decomposition product. 

Occurrence: A common though sparsely distributed 
accessory mineral in igneous rocks and as a magmatic 
segregation in igneous rocks. 

CORUNDUM. 
Composition : A1 2 3 . 

Criteria for determination in thin section : 
Form : Prisms, grains or plates. Rhombohedral 
cleavage may show. 

Optical Properties : Uniaxial and negative. 

Colorless or with patches or zones of blue. 

Refringence very high and surface rough, n 
1.7676 and 1.7594. 

Birefringence weak like quartz (0.082). Inter- 
ference colors middle of the first order, yellow to blue. 

Interference figure of basal section shows indis- 
tinct cross. 



DESCRIPTION OF ROCK-MAKING MINERALS 79 

Pleochroism marked when color is deep. Z is blue ; 

X is green. 

X axis coincides with crystallographic a. 

Occurrence : In crystalline metamorphic rocks, such 
as marble, gneisses, mica and chlorite schists, in perido- 
tites, in sands and gravels. 

Uses : Ruby, the red transparent variety, is valuable 
as a gem. Sapphire, which is likewise valued as a gem, 
is the blue transparent variety. Burma furnished the 
best rubies and Ceylon the best sapphires. It is also 
used as an abrasive. 

QUARTZ. 

Composition : Si0 2 , 

Criteria for determination in thin section : 
Form: Crystals usually prismatic, terminated by 
rhombohedrons. Allotriomorphic in granitoid rocks, 
rounded grains in clastic rocks. Rarely in distinct crys- 
tals in any rocks. May be mutually interpenetrated by 
an acid feldspar. Cleavage nearly always absent or 
difficult. 

Optical Properties: Uniaxial and positive. 

Colorless. By reflected light it may appear cloudy 
if it contains many inclusions. 

Refringence low. No relief and smooth surface. 
n = 1.553 and 1.554. 

Birefringence weak with interference colors of 
white or yellow in the middle of the first order (0.009) . 
Pleochroism absent. 

Extinction takes place, but is not distinctive, due 
to the absence of cleavage or crystallographic outline. 
Interference figure of a basal section shows a dark 
cross without any rings. 

Alteration : Does not alter, so that the fresh appear- 
ance of the mineral is an important aid in identification. 



80 OPTICAL MINERALOGY AND PETROGRAPHY 

Inclusions: Minute fluid or gas inclusions common 
in granitoid rocks. Not so abundant in porphyritic 
rocks. 

Occurrence: One of the most abundant minerals 
found in nature. It occurs in sedimentary, acid igneous, 
metamorphic rocks and veins. 

Differentiation: From sanadine, by uniaxial and 
positive character. 

From nephelite, by absence of hexagonal outline, 

stronger double refraction, and fresh undecomposed 

appearance. 

Uses: For ornamental purposes, for optical instru- 
ments, for glass-making, for pottery and porcelain, and 
as an abrasive. 

CALCITE. 

Composition : CaCOy. 
Criteria, for determination in thin section : 
Form: Grains and aggregates. May be fibrous or 
oolitic. Never in crystals in rocks. Polysynthetic twin- 
ning common, probably due in part to the grinding of 
the section. Shows in crossed nicols as a series of light 
and dark bands. Cleavage parallel to R appearing as 
many cracks. 

Optical Properties: Uniaxial and negative. 

Colorless when pure, but may appear colored by 
transmitted light, due to organic pigments. 

Refringence low. Relief not marked and surface 
smooth, n = 1.658 and 1.486. 

Birefringence very strong, with pale, iridescent 
interference colors of the fourth order (0.172). 

Extinction parallel to cleavage cracks when they 
appear. 

Pleochroism. No change of color observed, but 
absorption can be noted if section is not too thin. 



DESCRIPTION OF ROCK-MAKING MINERALS 81 

Interference figure of basal section shows distinct 
cross and rings. 
Inclusions of fluid frequent. 

Differentiation: From other carbonates, difficult 
except by microchemical tests. 

Occurrence: Abundant in sedimentary limestones 
and as a decomposition product in igneous rocks. Vein 
mineral often associated with ores as gangue. As trav- 
ertine and cave deposits. 

DOLOMITE. 

Composition: (Ca.Mg) CO 3 . 
Criteria for determination in thin section : 
Form: In rocks chiefly as crystals, usually unit R, 
with a tendency to curved surfaces. As dense homogene- 
ous aggregates showing tendency toward crystalline 
boundaries. 

Optical Properties : Uniaxial and negative. 

Similar to calcite, from which it may be differen- 
tiated by slightly higher relief (n = 1.682 and 1.503) 
and by tendency toward crystalline boundaries. 
Occurrence: As the essential constituent of dolo- 
mitic limestones, as a vein mineral, and as a secondary 
mineral in cavities in limestone. 

Uses : As limestones for building and ornamental 
purposes. Also for furnace linings. 

SIDERITE. 

Composition : FeC0 3 . 

Criteria for determination in thin section : 

Similar to calcite in form and optical properties. 

Absorption often distinct. 

Alteration : Changes readily on exposure to limonite 
and hematite. 

Differentiation: From calcite, by common associa- 
tion with limonite. 



82 OPTICAL MINERALOGY AND PETROGRAPHY 

From dolomite and magnesite, by common poly- 
synthetic twinning. It is the only mineral of the 
Calcite group with both indices of refraction higher 
than that of balsam except the rarer smithsonite and 
rhodocrosite. 

Occurrence : In limestone, clay iron-stone, clay slate, 
gneiss. Also in veins with metallic ores. 

APATITE. 

Composition: Ca,(Cl.F) (P0 4 ) 3 . 
Criteria for determination in thin section. 
Form : Minute, slender, hexagonal prisms, with reg- 
ular hexagonal boundaries. Grains. Clusters of crys- 
tals. Elongation parallel to a. Cleavage seldom 
observed. 

Optical Properties : Uniaxial and negative. 

Colorless usually in thin section. Sometimes gray, 
blue or brown, the color being irregularly distributed, 
due perhaps to microscopic inclusions. 

Refringence moderate. Relief more marked than 
of the associated colorless minerals, n 1.638 and 
1.634. < 

Birefringence: Weak, with interference colors 
grayish blue or white, of the lower first order (0.004) . 
Extinction parallel to c axis. 
Interference figure shows a cross without rings. 
Pleochroism absent for white crystals. Colored 
varieties weakly pleochroic. 
Alterations : Mineral always appears fresh. 
Differentiation: From nephelite, by occurring in 
smaller and longer crystals, and invariably fresher in 
appearance. 

From zircon, see under the latter mineral. 
From feldspars, when granular, by higher relief 
and the uniaxial interference figure. 



DESCRIPTION OF ROCK-MAKING MINERALS 83 

From quartz, in having a higher relief, weaker 

birefringence, and a negative sign. 

Occurrence : Widely distributed as an accessory con- 
stituent of igneous rocks and in crystalline schists. With 
metamorphic limestones. As a vein mineral in gabbro 
and in pegmatites. 

Uses: As a source of phosphates for fertilizers. 

NEPHELITE. 

Composition: NaAlSiO 4 . 
Criteria for determination in thin section : 
Form: Crystals thick, six-sided prisms with base 
prominent. Massive and in embedded grains. Cleavage 
imperfect, parallel to the prism of the first order and 
the base, better in partially altered sections. 
Optical Properties : Uniaxial and negative. 
Colorless in thin section. 

Refringence low. Relief absent, n = 1.546 and 
1.542. 

Birefringence very weak (0.005). Interference 
colors grayish white of the lower first order, a little 
lower than the feldspar colors. 

Extinction parallel to cleavage lines when they 
appear. 

Pleochroism absent. 

Interference figure is a broad cross without rings. 
Inclusions : Microscopic needles of augite, also fluid 
and gas generally in zones. 

Alteration : Readily to fibrous zeolites with stronger 
birefringence. 

Differentiation: From quartz, by weaker birefrin- 
gence, better hexagonal outline, and negative sign. 



84 OPTICAL MINERALOGY AND PETROGRAPHY 

From feldspars, by uniaxial character and absence 

of twinning. 

Occurrence: In nephelite syenites, phonolites, and 
rare soda-rich rocks. It is never associated with quartz, 
but often with orthoclase. 

TOURMALINE. 

Composition: R Si0 5 ? R chiefly Al, K, Fe, Ca, Mn, 
Mg, Li. 

Criteria for determination in thin section : 
Form: Columnar crystals, bunched or in radiating 
aggregates. Irregular cracks may appear, but no cleavage 
is seen. Cross section shows trigonal outline parallel to 
base. 

Optical Properties. Uniaxial and negative. 

Color: Varies, with grayish blue, brown, and 
black most common. Zonal structure may be shown 
by differences in color. 

Refringence medium. Conspicuous against the 
colorless rock constituents. Surface rough, n 
1.636. 

Birefringence quite strong (0.02), with bright 
interference colors of the upper first or lower second 
order. Often masked by strong absorption. 

Interference figure shows a sharp cross with a 
few rings. 

Extinction parallel to the c axis. 

X axis is parallel to the c axis. 

Pleochroism distinct even in light-colored vari- 
eties, increasing with the depth of the color. The 
greatest absorption takes place normal to the direc- 
tion of elongation of the mineral. Formula for Mg 
tourmaline Z is pale yellow. X is colorless. 

Absorption very marked. 
Alteration does not take place commonly. 



DESCRIPTION OF ROCK-MAKING MINERALS 85 

Differentiation: From hornblende, by absence of 
cleavage, and by the fact that the greatest absorption 
takes place at right angles to the longitudinal axis. 

Occurrence: Widely distributed in crystalline schists 
and gneisses, in crystalline limestones (New Jersey), in 
granite pegmatites and veins with copper minerals. It 
is a common product of contact metamorphism. 

Uses : Colored tourmaline is used as a gem. 



86 OPTICAL MINERALOGY AND PETROGRAPHY 



CHAPTER 7. 
Description of Minerals (Continued). 

ANISOTROPIC-BIAXIAL MINERALS. 

ORTHORHOMBIC. 

ANDALUSITE. 

Composition : Al 2 Si0 5 . 
Criteria for determination in thin section : 
Form : Prismatic crystals always more or less elon- 
gated parallel to the vertical axis, in rough or embedded 
crystals. Cleavage may show parallel to almost square 
prism. 

Optical Properties: Biaxial and negative. 
Color: Colorless to reddish. 
Refringence medium, n = 1.64 and 1.63. 
Birefringence weak (0.01). Interference colors, 
middle of the first order, white or yellow. 

Interference figure shows large optic angle. 
Extinction parallel to c. 

Pleochroism marked only in colored varieties, 
being reddish parallel to c, which is the direction of 
elongation or cleavage. Pleochroic halos often sur- 
round inclusions. 

Inclusions: Carbonaceous matter common, distrib- 
uted through the crystal in some geometrical form con- 
forming to the symmetry. 

Alteration: Readily to colorless mica. 
Differentiation: From diopside, by weaker bire- 
fringence and absence of extinction angles. 



DESCRIPTION OF ROCK-MAKING MINERALS 87 

Occurrence : In granitic eruptive rocks and in meta- 
morphosed sedimentary limestones. 

TOPAZ. 

Composition : Al 2 F,SiO 4 . 
Criteria for determination in thin section : 
Form : Colorless crystals of short prismatic habit. 
Cleavage perfect parallel to the base. 
Optical Properties: Biaxial and positive. 

Refringence medium, about the same as calcite. 
n = 1.617 and 1.607. 

Birefringence weak (0.01) , about the same as that 
of quartz with interference colors, middle of the first 
order white and yellow. 

Interference figure shows large optic angle. 
Extinction parallel to cleavage. 
Z axis parallel to c. 

Alteration : To kaolin or muscovite by loss of F and 
addition of water and alkalies. 

Differentiation : From quartz, by cleavage and biax- 
ial character. 

From andalusite, by its cleavage and its smaller 
optic angle. 

From orthoclase, by its higher relief, absence or 
rarity of twinning and extinction parallel with the 
cleavage. 

Occurrence: In contact metamorphic zones and in 
pegmatite, associated with cassiterite, fluorite, tourma- 
line, beryl, etc. In cavities in rhyolite. 
Uses : Occasionally as a gem. 

STAUROLITE. 

Composition: FeAl 5 (OH) (SiO ) 2 . 
Criteria for determination in thin section : 
Form: Short prisms twinned at 90 or 60 degrees. 
Cleavage variable, both prismatic and pinacoidal. 



88 OPTICAL MINERALOGY AND PETROGRAPHY 

Optical Properties : Biaxial and positive. 
Color : Yellowish to brown. 
Refringence rather high and surface rough. 
n = 1.746 and 1.736. 

Birefringence weak (0.01) with interference col- 
ors middle of first order white to yellow, about like 
quartz. 

Optic angle large. Plane of optic axis is parallel 
to 100. 

Pleochroism distinct but not strong, showing 
the darker color parallel to c, the direction of elon- 
gation (Z is golden yellow, Y is pale yellow, X is col- 
orless). 

Extinction parallel to cleavage or crystal outline. 
Inclusions of rutile, tourmaline, garnet and quartz 
occur, the latter abundantly. 

Alteration : To a green mica and chlorite. 
Differentiation: From titanite, by the fact that in 
convergent light the optic plane is shown to be in the 
longer diagonal of the cross section. 

Occurrence : In mica schists and phyllites associated 
with garnet, cyanite and andalusite. 
SERPENTINE. 

Composition : H 4 Mg 3 Si 2 O 9 . 
Criteria for determination in thin section: 
Form: Not known in crystal form. Fibrous or 
scaly masses with elongation parallel to c. Prismatic 
cleavage of 130 degrees seldom visible. 

Optical Properties: Biaxial and positive. 

Color in thin section: Pale green, yellow, or 
colorless. 

Refringence low, about the same as Canada bal- 
sam. No relief, and smooth surface, n = 1.54. 

Birefringence rather weak, with interference 



DESCRIPTION OF ROCK-MAKING MINERALS 89 

colors middle of the first order, gray, white or yellow. 
Between crossed nicols the aggregate structure is 
distinctly seen. Fine fibrous aggregates may appear 
isotropic (0.013). 

Pleochroism in thick sections distinct. Z is green 
or yellow. Y and X are greenish yellow to colorless. 
Optic plane parallel to 010. Optic angle is small. 
Extinction parallel. 

Differentiation: From chlorite, by its weaker ple- 
ochroism. 

From fibrous amphiboles, by much weaker bire- 
fringence, lower relief and parallel extinction. 
Fibrous structure and color indicative. 
Occurrence: As an alteration product of olivine, 
amphiboles, pyroxenes. The essential mineral in the 
metamorphic rock serpentine, derived from peridotite. 
A secondary occurrence in veins. 

Uses : As an ornamental stone. The fibrous variety 
forms a commercial asbestos. 

THE ORTHORHOMBIC PYROXENES. 

ENSTATITE AND HYPERSTHENE. 

ENSTATITE. 

Composition : MgSiO ;i . 

Criteria for determination in thin section : 

Form : Distinct crystals rare, prismatic. Columnar 
or fibrous structure parallel to c, characteristic of allo- 
triomorphic occurrences. Usually massive, fibrous or 
lamellar. Prism angle nearly 90 degrees. Twinning not 
as common as in the monoclinic pyroxenes. Prismatic 
cleavage distinct. 

Optical Properties : Biaxial and positive. 

Color: Colorless in thin sections. Bronzite, 

which is a variety of enstatite containing ferrous iron 



90 OPTICAL MINERALOGY. AND PETROGRAPHY 

in place of some of the magnesium, is colorless or 
nearly so, and shows strong pleochroism with X a 
pale yellow, Y a brownish yellow, and Z a bright 
green. 

Refringence high and surface rough, about the 
same as in the monoclinic pyroxenes, n = 1.665 and 
1.656. 

Birefringence weak (0.009) much weaker than 
the monoclinic pyroxenes. Interference colors low 
of first order. 

Interference figures not marked on account of the 
weak double refraction. 

Extinction parallel to cleavages, both pinacoidal 
and longitudinal prismatic, and bisecting angles of 
intersecting prismatic cleavages. 

Axial plane parallel to brachypinacoid, that is, 
parallel to the best cleavage. Axial angles large. 

Pleochroism weak or absent. 

Alteration: To serpentine by ordinary weathering. 
Also to uralite (a variety of hornblende) , but much less 
commonly than the monoclinic pyroxenes do. 

Differentiation: From hypersthene, by the optic 
sign and absence of distinct color and pleochroism. 

From the monoclinic pyroxenes, by parallel extinc- 
tion on vertical sections, and lower interference 
colors. 

Occurrence: A common constituent of basic igneous 
rocks as well as of serpentine derived from them. Also 
found in crystalline schists and in many meteorites. 
Bronzite contains about 10 per cent FeO and has a char- 
acteristic bronzy luster due to inclusions. 

HYPERSTHENE. 

Composition: (Mg, Fe) SiO.,. 

Criteria for determination in thin section : 



DESCRIPTION OF ROCK-MAKING MINERALS 91 

Form: Similar to enstatite. More often massive in 
lamellae. Elongated parallel to c. 

Optical Properties: Biaxial and negative. 
Color : Brownish to greenish. 
Refringence slightly higher than enstatite, due to 
increase in percentage of iron. 

Birefringence slightly stronger than enstatite, due 
to increase of iron. Weaker than monoclinic 
pyroxenes. 

Extinction same as enstatite. 
Axial plane parallel to brachypinacoid, i.e., parallel 
to the best cleavage. Optic angle about X becomes 
smaller with increase in iron content. 

Pleochroism distinct, increasing with increase in 
iron. Z is bright green, Y is yellowish brown, X is 
clear red. 

Inclusions: Gaseous, liquid, glassy. Also a reddish 
brown material regularly arranged, which gives it a 
peculiar submetallic bronze-like luster. They are 
believed to be inclusions of ilmenite, either primary or 
produced at depth under pressure by circulating waters 
acting along a cleavage or parting plane. 

Alteration: To a variety of serpentine called bas- 
tite, less commonly to uralite, occasionally to talc. 

Occurrence: Important constituent with plagioclase 
in basic igneous rocks, as norites and gabbros. Abundant 
in andesites. Found in meteorites. 

BASTITE. 

Bastite is a variety of serpentine to which the ortho- 
rhombic pyroxenes poor in iron alter frequently through 
the ordinary processes of weathering. It is geometrically 
oriented on the altered pyroxene, replacing crystal for 
crystal. It is composed of fibers often traversed by irreg- 
ular cracks. Cleavage traces of the two minerals coin- 



92 OPTICAL MINERALOGY AND PETROGRAPHY 

cide, but the optical properties differ. The pyroxene has 
a cleavage parallel to the trace of the optic plane. In 
bastite, the cleavage is perpendicular to the trace of the 
optic plane, and to the negative acute bisectrix. This 
is the surest distinction between them. 

Bastite is light yellowish or greenish. Refringence 
is less than that of the orthorhombic pyroxenes and about 
the same as Canada balsam. Birefringence is weak. 
Extinction is parallel to the fibres. Pleochroism is weak 
and seen only in thick sections. 

OLIVINE (CHRYSOLITE). 

Composition: (Mg. Fe) 2 Si0 4 . 
Criteria for determination in thin section : 
Form : Idiomorphic, or in grains or granular aggre- 
gates. Also massive. Longitudinal sections more or 
less lath-shaped with pointed ends. Outlines of crystals 
often corroded or rounded. Interpenetration twins 
occur. Cleavage distinct, parallel to brachypinacoid, 
less distinct parallel to macropinacoid, often made more 
visible by decomposition. An irregular fracturing is 
often conspicuous, especially where alteration to ser- 
pentine has commenced. Elongation usually parallel to c. 
Optical Properties : Biaxial and positive. 

Color: Nearly colorless, becoming reddish with 
high iron content. 

Refringence high. Relief marked and surface 
rough, n = 1.689 and 1.6535. 

Birefringence very strong, with interference colors 
of the second or third order, higher than the colors 
of augite (0.0359). 

Extinction always parallel to cleavage cracks. 
Axial plane parallel to the base, that is, at right 
angles to the general direction of elongation. Axial 
angle very large. 



DESCRIPTION OP ROCK-MAKING MINERALS 93 

Pleochroism absent except in reddish varieties. 

Inclusions : Magnetite, spinel, apatite, common. Also 
liquid or gas. 

Alteration: Alters readily. Altered forms are more 
frequently observed than the fresh. Serpentine is the 
commonest alteration product, with frequently a separa- 
tion of magnetite or hematite. The first alteration goes 
on along the cleavage and fracture cracks. 

It is easily altered by atmospheric weathering to car- 
bonates with limonite and opal or quartz. Calcite may 
usually be distinguished in this case. In contact with a 
feldspar it may alter to an amphibole by regional meta- 
morphism. The amphibole appears as a zone of pale 
green or colorless needles between the olivine and the 
feldspar. 

Differentiation : From light colored monoclinic pyrox- 
enes by parallel extinction, by poorer and unequal cleav- 
ages and stronger birefringence. Olivine should be easily 
recognized by its high refringence, a shagreen surface, no 
color, strong birefringence, and a large optic angle. 

Occurrence: Especially in basic igneous rocks, asso- 
ciated with augite, hypersthene, plagioclase, magnetite. 
An essential constituent of many meteorites, constituting 
the stony portion of the mass. 

Uses: The transparent variety is sometimes used as 
a gem under the name peridot. 

TALC. 

Composition : H,Mg ;( ( SiO :! ) 4 . 
Criteria for determination in thin section : 
Form: Colorless plates, elongated like rods. More 
rarely with round to hexagonal outline. May be arranged 
in rosettes. Often more or less compact foliated masses. 

Cleavage perfect parallel to the base like mica. Elonga- 
tion parallel to c. 



94 OPTICAL MINERALOGY AND PETROGRAPHY 

Optical Properties: Biaxial and negative. 
Colorless in thin section. 
Refringence moderate, n 1.589 and 1.539. 
Birefringence strong, with interference colors of 
the third order, like muscovite (0.05 to 0.035). 
Extinction parallel to basal cleavage lines. 
Plane of optic axes parallel to 100. Optic angles 
small. 

Differentiation: From muscovite, by its small optic 
angle. From sericite, by lower refringence. 

Occurrence: Most abundantly in crystalline schists, 
often forming rock masses, as soapstone. As a secondary 
mineral in basic igneous rocks, altering from olivine, en- 
statite, tremolite. 

Uses: For soap, talcum powder, French chalk, and 
in the manufacture of paper. 

NATROLITE. 

Composition: Na 2 ALSi.,0 10 . 2H 2 O. 
Criteria for determination in thin section: 
Form : Aggregates of colorless, fibrous crystals, often 
in interlacing groups or divergent. Prismatic angle 
nearly 90 degrees. Elongation parallel to c. Microscopic 
twinning on 110. 

Optical Properties: Biaxial and positive. 

Refringence very low, with no relief, n = 1.485 
and 1.473. 

Birefringence weak, though slightly stronger than 
quartz. 

Interference colors, middle of the first order yel- 
low, a little higher than quartz. 

Interference figure shows dark cross. Optic angle 
large. 

Plane of optic axis parallel to 010. 

Axis Z parallel to c. 



DESCRIPTION OF ROCK-MAKING MINERALS 95 

Extinction parallel to fibers. 

Occurrence : Never found as a primary mineral. As 
a secondary mineral in basic igneous rocks filling amyg- 
daloidal cavities. Common alteration product of sodalite, 
nephelite, and acid plagioclase. 

PYROXENE GROUP. 

Composition : The pyroxenes are metasilicates of cal- 
cium, magnesium, iron, or more complex silicates, often 
containing two or more bases, both bivalent and triva- 
lent. They are closely related to each other in crystallo- 
graphic and physical properties. They crystallize in the 
orthorhombic, monoclinic and triclinic systems. 
Criteria, for determination in thin section : 
Form : Fundamental form is a short prism with inter- 
facial angles of about 87 and 93 degrees. Distinct cleav- 
age occurs parallel to both prism faces. Twinning if 
present is parallel to 100. Elongation usually parallel 
to c. 

Optical Properties : Biaxial. Most species positive. 

Color: In thin section usually pale to colorless. 
Soda pyroxenes have a distinct green color. 

Refringence high. Relief distinct and surface 
rough, n = 1.68 to 1.72. 

Birefringence strong, being stronger in the pale 
or colorless pyroxenes with interference colors, 
bright tints of the second order (0.021 to 0.030). 

Extinction : Maximum angle from to 95 degrees, 
with common species varying between 30 and 54 de- 
grees. In sections showing parallel cleavage lines, 
parallel extinction in orthopinacoidal sections, and in 
all other sections an extinction angle is observed. The 
maximum extinction angle is large and is obtained 
only when the section of the crystal is parallel to the 
clinopinacoid. 



96 OPTICAL MINERALOGY AND PETROGRAPHY 

Optic plane is parallel to 010. 
Pleochroism weak or absent except in the soda 
pyroxene. 

Alteration : Alters readily to amphiboles. Described 
under each species. 

Differentiation: Pyroxenes may be distinguished 
from amphiboles by the following criteria : 

Pyroxenes. Amphiboles. 

Cleavage angle about 93 de- Cleavage angle about 124 

grees. degrees. 

Crystals short prismatic. Crystals long prismatic: 
Color usually weak. Color marked. 

Pleochroism weak. Pleochroism marked. 

Extinction angles 0-95 de- Extinction angles 0-25 de- 
grees, grees. 
Most species positive. Most species negative. 
Alter to amphiboles. Alter to chlorite, biotite, 

etc. 

MONOCLINIC MINERALS. 
Monoclinic Pyroxenes. 

Diopside. 
Diallage. 
Augite. 
Aegirite. 
DIOPSIDE. 

Composition: Ca(Mg, Fe) (Si0 3 ) 2 . 
Criteria, for determination in thin section : 
Form: Long, columnar crystals and grains. Often 
coarsely lamellar. Granular masses. Cleavage always 
distinct parallel to 110 in two directions nearly at right 
angles to each other. Parting parallel to the base, yield- 
ing fine twinning lamellae in this direction. Elongation 
parallel to c. 



DESCRIPTION OF ROCK-MAKING MINERALS 97 

Optical Properties : Biaxial and positive. 

Colorless usually in thin section ; but with increase 
in iron, color becomes distinctly greenish. 

Refringence increases with increase in iron con- 
tent, n = 1.7026 and 1.6727. 

Birefringence decreases with increase in iron con- 
tent (0.0299). 

Interference colors bright, of the second order. 
Interference figure is an axial bar with concentric 
rings. 

Extinction angle from 20 to 30 degrees. 
Plane of the optic axes parallel to 010. 
Dispersion weak. 

Inclusion: Gaseous, liquid, or glassy, arranged in 
zones. 

Alteration : Most commonly to serpentine or to an 
aggregate of serpentine and chlorite, often with calcite 
and quartz. Also to actinolite or hornblende, alteration 
starting around the periphery or along cleavage cracks. 
Differentiation: From augite, by less dispersion, 
consequently better extinction in white light. 

From segirite and spodumene, by the extinction 
angle in the vertical zone. 

From hypersthene, by the absence of pleochroism. 
From orthorhombic pyroxenes, by the extinction 
angle and the higher order of colors. 
Occurrence: In crystalline limestones as a contact 
mineral with garnet. In many igneous rocks as granites, 
diorites, syenites, gabbros, and peridotite. In metamor- 
phic rocks. 

DIALLAGE. 

A variety of diopside showing well developed parting 
parallel to 110, generally showing a fibrous tex- 
ture parallel to c. Color usually brown, with pleochroism 



98 OPTICAL MINERALOGY AND PETROGRAPHY 

as follows: Z is greenish, Y is brownish or reddish 
brown, X is greenish. It contains inclusions like those 
of hypersthene, which gives it a bronze-like luster. 

AUGITE. 

Composition: mCaMg (SiO ;! ) 2 . n(Mg, Fe) (Al,Fe) L . 
Si0 6 . 

Criteria for determination in thin section : 
Form : Crystals short thick prisms coarsely lamellar, 
parallel to 001 or 100. Granular. Twinning common, giv- 
ing polysynthetic lamellae parallel to 100. Cleavage 
imperfect, but distinct in two directions parallel to 110 
nearly at right angles. Elongation parallel to c. 
Optical Properties : Biaxial and positive. 

Color green, greenish black, brown. Rarely 
yellow. 

Refringence high. High relief and rough surface. 
n = 1.733 and 1.712. 

Birefringence rather strong, being stronger in the 
pale or colorless pyroxenes. Interference colors are 
bright tints of the second order (0.021) . 

Interference figures distinct on account of the 
strong birefringence. Axial angles large. 

Optic plane parallel to the clinopinacoid (010). 
Extinction angle: Maximum from 38 to 51 
degrees, which is obtained when the section of the 
crystal is parallel to the clinopinacoid (010), varying 
from these angles to degrees when the section is 
parallel to the orthopinacoid (100). 

Pleochroism usually absent or weak unless rich in 
iron, in which case Z is greenish, Y is brownish to red- 
dish brown, and X is green. 

Inclusions: Gaseous, liquid or glassy, sometimes 
arranged in zones. 



DESCRIPTION OF ROCK-MAKING MINERALS 99 

Alteration: Most commonly to uralite, a variety 
of hornblende, either crystal for crystal, or to a fibrous 
aggregate of uralite. The alteration begins around the 
periphery of the crystal or along cleavage cracks. It 
may alter to biotite and then to chlorite, or directly to 
chlorite, sometimes forming calcite, quartz or epidote 
simultaneously. 

Differentiation : From diopside, see under the latter. 
From segirite and spodumene in the extinction 
angle in the vertical zone. 

From amphiboles, see under Pyroxene group. 
From epidote, by the fact that the plane of the 
optic axis is parallel to the longitudinal axis and cleav- 
age cracks. 

Occurrence: Abundant in igneous rocks, but found 
also in metamorphic rocks. Occurs in some stony 
meteorites. 

^GIRITE (ACMITE). 
Composition: Na Fe (SiO 3 ) 2 . 
Criteria for determination in thin section : 
Form: In crystal form, similar to augite, although 
often longer or acicular. Cleavage parallel to 110, more 
distinct than in augite, almost at right angles. Part- 
ing parallel to 100. Elongation parallel to c. 
Optical Properties : Biaxial and negative. 

Color in thin section greenish or brownish. 
Refringence high, with high relief and rough sur- 
face, n = 1.8126 and 1.762. 

Birefringence quite strong, with bright tints of 
the second order, although it is stronger in the pale 
or colorless varieties of pyroxene (0.0496). 

Interference figures distinct on account of strong 
birefringence. Optic angle is large. 

Optic plane parallel to the clinopinacoid (010). 



100 OPTICAL MINERALOGY AND PETROGRAPHY 

Extinction angle: About 5 degrees. 
Pleochroism marked. Z is yellowish green, Y is 
olive green, X is dark grass green. 
Alteration : To analcite and to the iron oxides. 
Differentiation : From the amphiboles, see under 
pyroxene group. 

From other monoclinic pyroxenes, by the very 
small extinction angle, the negative sign, the stronger 
birefringence and marked pleochroism. 
Occurrence : In pegmatite veins, in soda-rich igneous 
rocks, as nephelite, syenites, phonolites and soda- 
granites. 

AMPHIBOLE GROUP. 

Composition: The minerals of the amphibole group 
are orthorhombic, monoclinic, and triclinic silicates of 
magnesium, calcium iron or sodium, with aluminum or 
ferric iron in some cases. 

Criteria for determination in thin section : 
Form: Crystals usually prismatic, elongated parallel 
to c, possessing very marked and regular prismatic cleav- 
ages, varying little from 124 degrees between the cleav- 
age faces. Twinning common parallel to 100. 

Optical Properties: Biaxial. Most species negative. 
Color in thin section are green, brown, blue, yel- 
low or colorless. 

Refringence averages less than the pyroxenes, 
increasing with increase of iron. Relief distinct, 
n = 1.621 to 1.642. 

Birefringence quite strong, but a little weaker 
than in the pyroxenes. Interference colors are bright 
tints of the second order. May be masked by strong 
absorption (0.019 to 0.027). 

Extinction : Maximum angle from to 25 degrees. 



DESCRIPTION OF ROCK-MAKING MINERALS 101 

In the monoclinic amphiboles the axis Z makes an 
angle with the vertical crystallographic axis c, which 
varies from to 22 degrees, except in uncommon 
species. Elongation therefore positive. 

Pleochroism distinct and intense in the colored 
varieties. 

Absorption very marked, being greatest in the 
general direction of the cleavage lines in the longi- 
tudinal sections (parallel to Z). 

Optic plane parallel to 010 in monoclinic 
amphiboles. 

Inclusions: Iron ores, apatite, etc. 
Alteration : Alter readily to chlorite, biotite, sericite, 
epidote, calcite, talc, etc., the process being gradual, and 
usually beginning along the edges and the cleavages of 
the amphibole until all trace of the original mineral is 
lost. 

Differentiation: Amphiboles may be distinguished 
from the pyroxenes by the following criteria : 

Amphiboles. Pyroxenes. 

Cleavage angle about 124 Cleavage angle about 93 

degrees. degrees. 
Crystals long prismatic. Crystals short prismatic. 
Color marked. Color usually weak. 
Pleochroism marked. Pleochroism weak. 
Extinction angles, 0-25 de- Extinction angles, 0-95 de- 
grees, grees. 
Most species negative. Most species positive. 
Alter to chlorite, biotite, Alter to amphiboles. 
etc. 

Occurrence: In all classes of eruptive rocks and in 

many metamorphic rocks. Often formed by alteration 
from pyroxenes. 



102 OPTICAL MINERALOGY AND PETROGRAPHY 

Monoclinic Amphiboles: 

Tremolite. 
Actinolite. 
Hornblende. 
Riebeckite. 

TREMOLITE. 

Composition : Ca Mg, ( Si0 3 ) 4 . 
Criteria for determination in thin section : 
Form : Crystals long-bladed or short prismatic. Often 
fibrous or acicular. Perfect prismatic cleavage at an 
angle of about 124 degrees. Cleavage sometimes distinct, 
parallel to 010 and 100. Transverse fracture frequent. 
Cleavage more perfect than in pyroxenes. 
Optical properties: Biaxial and negative. 
Colorless in thin section. 

Refringence high, with distinct relief, but not as 
marked as in the pyroxenes, n = 1.634 and 1.6065. 

Birefringence quite strong, but a little weaker 
than in pyroxenes (0.0275). 
Optic plane parallel to 010. 

Maximum extinction angle is 18 to 16 degrees in 
vertical zone. 

Dispersion weak. 

Inclusions of carbonaceous matter and biotite in 
tremolite of metamorphic rocks. 

Alteration: To talc, beginning along cleavage lines. 
Also to calcite. 

Differentiation : From hornblende, by light color. It 
has the lowest index of refraction found in monoclinic 
amphiboles. 

From pyroxenes, see under Amphibole group. 
Occurrence : In schists, contact rocks, and veins. 
Uses: Fibrous varieties sometimes used as asbestos. 
As jade, sometimes used for ornamental purposes. 



DESCRIPTION OP ROCK-MAKING MINERALS 103 

ACTING-LITE. 

Composition : Ca ( Mg, Fe) 3 ( SiO ;! ) 4 . 
Criteria for determination in thin section: 
Form : Similar to tremolite. 
Optical Properties: Biaxial and negative. 

Color in thin section pale to dark green, depending 
upon the percentage of iron. 

Refringence and birefringence similar to tremo- 
lite (0.025). 

Maximum extinction angle in vertical zone is 15 
degrees. 

Dispersion weak. 

Pleochroism pronounced, and absorption marked, 
being greatest in the general direction of the cleavage 
lines in longitudinal sections. Z is pale to dark green, 
Y is greenish yellow, X is very pale yellow. 
Inclusions : Similar to tremolite. 
Alteration : To chlorite, epidote, talc, etc. 
Occurrence : Same as tremolite, with which it is often 
associated. Uralite is the name given to the amphibole 
to which the pyroxenes frequently alter. It usually cor- 
responds to actinolite. 

HORNBLENDE. 

Composition: mCa(Mg, Fe) C) (SiO,) 4 . n(Al, Fe) 
(P,OH)SiO s . 

Criteria for determination in thin section : 
Form : Prismatic elongated, parallel to the vertical 
axis, sometimes fibrous. Prismatic cleavage perfect, 
making the characteristic angle of 124 degrees. Parting 
and polysynthetic twinning are sometimes present, par- 
allel to 100 or 001. Cross sections may be acutely 
rhombic, with acute angles truncated, hence six-sided, 
whereas the pyroxenes are usually eight-sided. Longi- 
tudinal sections lath-shaped. Zonal structure occurs fre- 



104 OPTICAL MINERALOGY AND PETROGRAPHY 

quently in the brown hornblende. Twinning frequently 
parallel to the orthopinacoid. 

Optical Properties: Biaxial and negative. 

Colorless, gray, green, greenish blue, brown or 
black. 

Refringence high, with distinct relief, n = 1.653 
and 1.629. 

Birefringence quite strong, being strongest in 
the basaltic hornblende. In common hornblende, 
n= (0.024). In basaltic hornblende, n= (0.072). 
Interference colors, bright tints of the second order, 
often masked by strong absorption. 

Maximum extinction angle in common hornblende 

20 degrees, in basaltic hornblende from 1 to 2 degrees. 

Optic plane parallel to 010, in which face Z makes 

a variable angle with the axis c in the obtuse angle 

Beta. 

Dispersion distinct. 

Pleochroism : Z pale green, Y pale brown, X clear 
brown, for common hornblende. 
Inclusions abundant but not characteristic. Rutile 
common. 

Alteration : By ordinary weathering to chlorite often 
accompanied by epidote, calcite and quartz. Sometimes 
to biotite. By heat to augite. 

Differentiation : From other amphiboles, by stronger 
color and pleochroism, higher interference colors. 

From pyroxenes, see under Amphibole group. 
Occurrence: Widespread in igneous, regional meta- 
morphic and contact rocks. Hornblende schists. 
RIEBECKITE. 

Composition: nNaFeSi 2 O 6 . FeSi0 3 . 
Criteria for determination in thin section : 
Form : Similar to hornblende. 



DESCRIPTION OF ROCK-MAKING MINERALS 105 

Optical Properties : Biaxial and negative. 

Color : Dark blue to black. 

Refringence same as hornblende. 

Birefringence weak (0.005). Interference colors 
masked by absorption. 

Optic angle parallel to 010. Angle large. 

Pleochroism intense. Z is yellowish green, Y is 
blue, X is indigo blue, nearly black. 

Absorption marked. 

Dispersion strong. 

Differentiation: Characterized by intense color and 
pleochroism, strong dispersion and pronounced color. 

Occurrence : In soda-rich igneous rocks and in some 
metamorphic rocks. 

MICA GROUP. 
Muscovite. 
Sericite. 
Biotite. 
Lepidolite. 
Phlogopite. 

Criteria for determination in thin section : 
Form: Monoclinic or pseudohexagonal. As scales, 
which may be notched or jagged, with lateral sections 
lath-shaped. As shreds, characterized by perfect basal 
cleavage giving thin laminae. Plates of hexagonal out- 
line with the planes 001, 110, 010, with angles of 60 
and 90 degrees. Twinning common after the mica law 
in a plane perpendicular to 001 and practically parallel 
to 110. Zonal structure common in the dark varieties. 
Elongation parallel to the cleavage and the c axis. 
Optical Properties : Biaxial and negative. 

Colors given under each variety. 
Refringence medium. Relief distinct. 

Birefringence very strong, particularly in the col- 



106 OPTICAL MINERALOGY AND PETROGRAPHY 

ored micas, varying from 0.037 to 0.05. Interference 
colors of the third order, which may be very brilliant 
in thin sections of colorless mica, often appearing 
iridescent. Occasionally masked by strong absorption. 
Extinction about parallel to the cleavage lines. 
Very small extinction angles may be noticed in biotite. 
Absorption strong in colored micas. 
Optic angle large in white micas and small in the 
ferro-magnesian varieties, appearing almost uniaxial. 
Differentiation: Characterized by distinct relief, 
strong birefringence (chlorite has weak), one perfect 
cleavage marked by parallel, fine lines, practically par- 
allel extinction, mottled appearance between crossed 
nicols, maximum extinction in colored varieties parallel 
to the vibration plane of the polarizer. 

MUSCOVITE. 

Composition: H 2 (K, Na) AL(Si0 4 ),. 

Criteria for determination in thin section : 

See also under Mica group. 

Colorless in thin section. 

Inclusions: Not as common as in biotite. Zircon, 
apatite, spinel, garnet, quartz, and magnetite. 

Alteration : By ordinary weathering to sericite, ser- 
pentine, talc. 

Occurrence: Most common of the micas. Normal 
constituent of igneous rocks, especially granites. Abun- 
dant in gneisses and schists. Present in veins. Occurs 
as an alteration product of the feldspars, nephelite, etc. 

Differentiation: From talc, by large optic angle. 
From kaolinite, by strong birefringence. 
From other micas, by being colorless in thin 

section. 

From chlorite, by strong birefringence and lack 

of color. 



DESCRIPTION OF ROCK-MAKING MINERALS 107 

SERICITE. 

Sericite is a fine, scaly or fibrous variety of muscovite, 
with a greater degree of hydration. It is nearly uniaxial 
in character, with a small optic angle. 

BIOTITE. 
Pseudohexagonal. 

Composition: (K,H) 2 (Mg, Fe) 2 (Al, Fe) 2 (SiO 4 ) 3 . 
Criteria for determination in thin section : 
See also under Mica group. 
Optical Properties: 

Color : Black, green, brown, red, yellow. 
Angle of optic axes almost degrees in most bio- 
tite of igneous rocks. 

Birefringence increases with increase in iron- 
content. 

Absorption marked. 

Pleochroism distinct. Z is dark to opaque brown, 
Y is the same, X is pure yellow. 
Inclusions : Apatite and zircon common. Pleochroic 
halos abundant about inclusions. 

Alteration: Reaily to chlorite, often accompanied 
by the formation of calcite, epidote and quartz. 

Differentiation: From alkaline micas by small optic 
angle color and distinct pleochroism. 

From chlorite, by strong birefringence and color. 
From hornblende, by extinction parallel to cleav- 
age and almost uniaxial interference figures in con- 
vergent light. 

Occurrence : Important constituent of many igneous 
rocks, gneisses and schists. Developed by regional and 
contact metamorphism. 

LEPIDOLITE. 

Composition: (Li, K) A1(F, OH), Al(Si0 3 ) 3 . 
Criteria for determination in thin section : 



108 OPTICAL MINERALOGY AND PETROGRAPHY 

See also under Mica group. 

Colorless in thin section. 

Pleochroism distinct. Z and Y pink ; X, colorless. 

Differentiation : Microscopically indistinguishable 
from muscovite. Pink color is believed to be due to 
traces of manganese. 

Occurrence : In veins and dikes in granite, associated 
with cassiterite, tourmaline, etc. 

Uses : As a source of lithium salts. 

PHLOGOPITE. 

Composition: (K, H) 3 (Mg, F), Mg ; ,Al(SiO 4 ) 3 . 

Criteria for determination in thin section : 

See also under Mica group. 

Color: Brown, brownish red, green, yellow. 

Pleochroism, Z and Y are brownish yellow, X is col- 
orless. 

Inclusions: Hematite, rutile and tourmaline are 
common. 

Differentiation: From minerals of other groups, 
same as biotite. 

From muscovite, paragonite and lepidolite by color. 
From biotite, by mode of occurrence. 

Occurrence: Only in crystalline limestones, dolo- 
mites, and serpentines, associated with spinel, graphite, 
etc. Absent in igneous rocks. 

Uses: As an insulator in electrical apparatus. 

CHLORITE GROUP. 
Penninite. 
Clinochlore. 

Character : Similar to the micas, with perfect cleav- 
age parallel to 001, the basal pinacoid. This cleavage 
may not be noticed in fibrous or secondary chlorite. 
Aggregates of small, flat scales of irregular outline, usu- 



DESCRIPTION OF ROCK-MAKING MINERALS 109 

ally with a laminated structure. Often in minute grains 
as a pigment in other minerals. Twinning common after 
the base and after the mica law. 

Criteria for determination in thin section : 

Color: Characteristically green, due to iron protox- 
ide, varying from greenish white to dark green. 

Refringence low. No relief. In penninite, n is 1.579 
and 1.576. In clinochlore, n is 1.596 and 1.585. 

Birefrigence usually weak, with interference colors 
of the low first order gray and bluish gray. Penninite 
(0.002). Clinochlore (0.011). 

Extinction : Plates parallel to cleavage show at 
times isotropic characteristics. In other sections, extinc- 
tion is apparently parallel to the cleavage. Clinochlore 
occasionally shows perceptible extinction angles. 

Pleochroism present in all chlorites in green and yel- 
low tints, the green being parallel to the cleavage. 

Maximum absorption always in the direction of the 
cleavage. 

Differentiation: From serpentine, by greater ple- 
ochroism. From mica, by weaker birefringence. Charac- 
teristics are pale green color, distinct pleochroism, low 
relief and weak birefringence. 

Occurrence: Widely distributed, forming essential 
constituent of chlorite schist. Occurs secondary in igne- 
ous and metamorphic rocks, from the micas, amphiboles, 
pyroxenes, and garnets. 

PENNINITE. 
Pseudorhombohedral. 
Composition : H s (Mg, Fe) , Al 2 Si,O ls . 
Differentiation from clinochlore: Nearly uniaxial 
character, negative sign, very weak birefringence, par- 
allel extinction. 



110 OPTICAL MINERALOGY AND PETROGRAPHY 

CLINOCHLORE. 

Composition : Same as penninite. 

Differentiation from penninite: Distinctly biaxial, 
positive sign, higher birefringence, occasionally oblique 
extinction, common polysynthetic twinning. 

EPIDOTE GROUP. 

Composition: Ca 2 (Al, Fe) 2 (Al, OH) (Si0 4 ) 3 . 
Criteria for determination in thin section : 
Form: Columnar crystals, nearly always elongated, 
parallel to the b axis. Fibrous, massive, or in irregular 
grains as aggregates. Twinning common parallel to 100. 
Cleavage parallel to the basal pinacoid, imperfect parallel 
to the orthopinacoid. Basal cleavage cracks not very 
numerous, and appear parallel to the general direction 
of elongation. 

Optical Properties: Biaxial and negative. 
Colorless to orange yellow in thin section. 
Refringence high with rough surface, n = 1.767 
and 1.730. 

Birefringence variable, often strong, with high 
interference colors. Variable in a single crystal 
(0.037). 

Extinction parallel to cleavage in elongated sec- 
tions. In other sections, extinction angle varies from 
to 28 degrees. 

Interference figure of cleavage flakes show an axial 
bar with concentric rings. Axial plane at right 
angles to the elongation of the crystal. Axial angles 
are large. 

Pleochroism: Z is colorless, yellowish green, 
pink; Y is pale blue to greenish yellow; X is colorless, 
lemon yellow, pale green. 

Alteration : Epidote is very resistant to weathering. 
Differentiation: From light colored monoclinic 



DESCRIPTION OF ROCK-MAKING MINERALS 111 

pyroxenes, by having optic plane at right angles to the 
principal cleavage cracks, which are parallel to the direc- 
tion of elongation. Epidote is characterized by form and 
color, high refringence, parallel extinction in longitudinal 
sections, strong birefringence variable in a single crystal. 
Occurrence: Very common, especially in schists and 
in zones produced by contact metamorphism between 
granites and limestones. Also as an alteration product 
of the ferro-magnesian minerals and feldspars in igneous 
rocks. 

ZOISITE. 

(Orthorhombic member of the Epidote group.) 
Composition : Same as epidote without the iron. 
Criteria for determination in thin section: 
Form: Prismatic crystals or granular aggregates. 
Lamellar, fibrous or in compact masses. Perfect cleav- 
age, parallel to 010; difficult, parallel to 100. Longer 
individuals show transverse parting. Microscopic twin- 
ning in polysynthetic bands occur. 

Optical Properties : Biaxial and positive. 

Colorless to yellow tints. Usually lacks color. 
Refringence high, with rough surface, n = 1.702 
and 1.697. 

Birefringence weak, with grayish or whitish inter- 
ference colors (0.005) . 

Extinction always parallel. 

Differentiation: From epidote, by its lack of color 
and its weaker birefringence. It is characterized by 
parallel extinction, high relief, very weak birefringence, 
strong dispersion. 

Occurrence: In crystalline schists, associated with 
amphibole, particularly hornblende. In igneous rocks, 
as an alteration of the feldspars. In veins in altered 
basic igneous rocks with quartz. 



112 OPTICAL MINERALOGY AND PETROGRAPHY 

KAOLINITE. 

Composition : H 4 ALSi 2 9 . 
Criteria for determination in thin section: 
Form: Pseudohexagonal, in thin plates or scales. 
Usually in clay-like masses. 

Optical Properties. Negative. 

Colorless in thin section. Aggregates are cloudy. 
Refringence low with no relief, n = 1.563. 
Birefringence weak (0.007). 

Differentiation: From muscovite and talc by weak 
birefringence. 

Occurrence: Kaolinite is the most common second- 
ary mineral. It is derived from the feldspars by ordi- 
nary weathering. Occurs in large sedimentary clay 
masses as a result of the decomposition of aluminous 
silicates. 

Uses: It is used in the manufacture of porcelain, 
pottery, and china. 

TITANITE (SPHENE). 

Composition : CaTiSi0 5 . 

Criteria for determination in thin section : 

Form: In detached crystals and in disseminated 
grains. Often wedge-shaped when primary, and irreg- 
ular grains when secondary. Flattened parallel to the 
base. Elongated parallel to a or c. Cleavage imperfect, 
parallel to the prism, appearing as a few rough cracks. 
Cleavage rarely observed in secondary forms. Twinning 
seen only between crossed nicols, the twinning boundaries 
bisecting the acute angles of the rhombs. 

Optical Properties : 

Colorless, brownish or yellowish. 

Refringence high, with rough surface, n = 2.009 

and 1.888. 



DESCRIPTION OF ROCK-MAKING MINERALS 113 

Birefringence extremely strong, with interference 
colors of a high order, like those of calcite (0.1214). 
Pleochroism strong in deeply colored varieties, 
appearing yellowish, parallel to a and reddish par- 
allel to c. 

Optic plane parallel to 010, with a small optic 
angle. 
Inclusions: Often grouped about the center of the 

crystal. 

Alteration: To a light yellow amorphous mass with 

calcite. 

Differentiation: From staurolite, by the fact that 

the optic plane is in the shorter diagonal of the cross 

section instead of in the longer. It is characterized by 

high relief, extreme birefringence, biaxial character and 

positive sign. 

Occurrence: Widely distributed as an accessory 

mineral in igneous rocks. Occurs in schists and gneisses. 

Is found as a secondary product called leucoxene, derived 

from ilmenite in basic igneous rocks. 

FELDSPAR GROUP. 

Monoclinic Feldspar. 
Orthoclase. 

Triclinic Feldspars. 

Microcline. 

Albite. 

Oligoclase. 

Labradorite. 

Anorthite. 

Composition : Silicates of aluminum with potassium, 
sodium, calcium, rarely barium. 

Form: Crystal forms are similar, often short pris- 



114 OPTICAL MINERALOGY AND PETROGRAPHY 

matic, somewhat flattened, parallel to 010. Narrow 
bands of albite, intergrown with orthoclase or microcline, 
forming "perthite" common. Plagioclase feldspars are 
triclinic, but angle alpha varies little from 90 degrees. 

Cleavage : Perfect parallel to the basal pinacoid and 
almost as perfect parallel to the clinopinacoid. Cleavage 
cracks usually noticed only in very thin sections. The 
two cleavages intersect at 90 degrees in orthoclase and 
at 93 or 94 degrees in the plagioclase feldspars. The 
cleavage is not as distinct as the cleavage of mica or 
hornblende. 

Twinning: Twinning common in the feldspars fol- 
lowing the Carlsbad, Mannebach, Baveno, Albite and 
Pericline laws. 

CARLSBAD TWINNING is the simplest type of feldspar 
twinning, and it occurs in both monoclinic and triclinic 
varieties, in the latter case causing confusion with other 
types of twinning. The twinning plane is the orthopina- 
coid and the composition face is the clinopinacoid. Carls- 
bad twins always consist of two individuals, a fact which 
may be used to differentiate between the plagioclase feld- 
spars and orthoclase. 

MANNEBACH TWINNING: The basal pinacoid is the 
composition face and twinning plane. This type of twin- 
ning is not common. 

BAVENO TWINNING : The twinning plane is the clino- 
dome to which the twinning axis is normal. Sections 
cutting such a twin show square or rhombohedral out- 
lines, the cleavages being parallel to the sides. 

ALBITE AND PERICLINE TWINNING are especially com- 
mon on the plagioclase feldspars and are used as a means 
of identification. They are usually visible to the naked 
eye. In thin sections they appear as polysynthetic stria- 
tions in narrow alternating light and dark bands, which 



DESCRIPTION OF ROCK-MAKING MINERALS 



115 



extinguish alternately upon being rotated. In the albite 
type of twin, the twinning axis is normal to the clinopina- 
coid. Hence the lamellae are parallel to the clinopinacoid 
and the striations are visible only on the basal pinacoid 
and the orthopinacoid. 

In the pericline twinning, the twinning axis is parallel 
to 6. Therefore, the pericline striations are visible on all 
faces of the crystal. It is obvious that if twinning occurs 
on the clinopinacoid, it must be of the pericline type. In 
thin section, the pericline twinning is visible in any sec- 
tion except a section cut parallel to the composition face. 




Fig. 19. Triclinic feldspar form, showing the positions of the 
characteristic albite and pericline striations. 

Differentiation of the Feldspars by Twinning. 

Orthoclase occurs in simple twins, after the Carlsbad, 
Baveno and Mannebach laws, but never in polysynthetic 
twins. 

Microcline is always polysynthetically twinned in two 
directions, a combination of albite and pericline twinning 
producing a rectangular crosshatching between crossed 
nicols. 



116 OPTICAL MINERALOGY AND PETROGRAPHY 

Plagioclase feldspars practically always show a poly- 
synthetic twinning, after the albite law. 

Albite shows twinning lines that are fine and far 
apart, irregular and interrupted. 

Oligoclase shows twinning lines that are clear and 
of regular widths. 

Labradorite shows twinning lamellae which are clear 
and definite, but the width often varies from one lamella 
to another. 

Anorthite shows twinning lamellae which are broad 
and regular, after the albite law, while those of the peri- 
cline law are distributed only in certain of the albite 
bands. 

Optical Properties. The optic plane containing the 
optic axes and the bisectrices is the chief optic element. 
Its position in each of the feldspars has definite relations 
to the cleavage, external faces, axes, and the positions 
of the albite twinning. In orthoclase and microcline, 
for example, the optic plane is almost parallel to the 
basal pinacoid, hence agrees with the direction of the 
most perfect cleavage. Z is perpendicular to the clino- 
pinacoid. X lies in the plane of the clinopinacoid almost 
parallel to the base, varying about 5 degrees in 
microcline. 

Refringence is low, similar to that of quartz. The 
Becke test is advised. 

Birefringence is weak, similar to that of quartz. 

TABLE OF REFRINGENCE OF THE FELDSPARS. 

n n n 

g m p 

Orthoclase . . . 1.526 1.5237 1.518 

Microcline . . . - . . 1.5296 1.5264 1.5224 

Albite .... 1.54 1.534 1.531 

Oligoclase .... 1.5469 1.5431 1.5389 

Labradorite . . . 1.5625 1.5578 1.5548 

Anorthite .... 1.5884 1.5837 1.5757 



DESCRIPTION OF ROCK-MAKING MINERALS 117 

TABLE OF BIREFRINGENCE OF THE FELDSPARS. 

Section Normal Parallel to 
Normal to X to Z Optic Plane 

n n n n n n 

g in m p g p 

Orthoclase . . . 0.0023 0.0047 0.007 

Microcline .... 0.0032 0.004 0.0072 

Albite . . . . 0.006 0.003 0.009 

Oligoclase .... 0.0038 0.0042 0.008 

Labradorite . . . 0.0047 0.003 0.0077 

Anorthite .... 0.0047 0.008 0.0127 

Alteration: In zone of weathering to kaolinite, 
quartz, and calcite. The alteration of the feldspars to 
kaolinite or to other closely associated hydrous aluminum 
silicates is the ordinary method of origin of clay, and 
takes place more frequently in the acid than in the basic 
feldspars by a leaching out of the potash and hydration. 
The alteration begins along cleavage cracks, and finally 
spreads over the entire feldspar, causing it to appear 
opaque or cloudy. The kaolinite is usually in small flakes. 

The alteration of the acid feldspars to sericite, a 
variety of muscovite, is common, the alteration taking 
place first along cleavage cracks. This is usually 
accomplished through the agency of hot solutions. 

The basic feldspars frequently alter to chlorite, also 
to epidote associated with quartz and calcite. 

Occurrence: Feldspars are the most abundant and 
the most widely distributed minerals of the earth's crust, 
occur abundantly in metamorphic rocks, and frequently 
in sedimentary rocks. Also in veins. 

Differentiation : From quartz, by presence of cleavage 
and twinning, and biaxial character. Characterized by 
frequency of occurrence in practically all conditions, low 
refringence, and weak birefringence. Also by readiness 
with which they alter. To distinguish one feldspar from 
another the twinning, extinction angles, optic sign and 
refringence as determined by the Becke test are aids. 



118 OPTICAL MINERALOGY AND PETROGRAPHY 

To distiguish one plagioclase feldspar from another, 
several practical methods have been devised. 

1. EXTINCTION ANGLES ON BASE AND BRACHY- 

PINACOD). 

Schuster established relations existing between the 
extinction angles on the base and the brachypinacoid. 
The prevalence of favorable cleavages aids in this deter- 
mination. As these minerals are all triclinic, extinction 
takes place in all sections unsymmetrically with respect 
to crystallographic, twinning or cleavage lines. Conse- 
quently, extinction angles will always be observed. When 
the extinction angles on both the basal pinacoid and 
the brachypinacoid are large, anorthite is in all prob- 
ability the mineral observed. When the angles are both 
small, the feldspar is oligoclase. Albite and labradorite 
show intermediate extinction angles. Orthoclase has 
extinction on the basal pinacoid of from 5 to 9 degrees. 

The extinction angles given in the following table 
are marked plus or minus. The angles on the base and 
brachypinacoid are marked plus when the direction of 
extinction has apparently moved, as the hands of a watch, 
with reference to the upper right-hand edge of the crys- 
tal, between the base and pinacoid. The angles are 
marked minus when the reverse is true. 



ANGLES. 

Section parallel to base Section parallel to brach- 

measured from trace of ypinacoid measured from 

pinacoidal cleavage. trace of basal cleavage. 

Albite ............ 4 Albite ............ 20 

Oligoclase ......... 2 Oligoclase ........ 7 

Labradorite ...... 51/2 Labradorite ...... 20 

Anorthite ........ 37 Anorthite . .. 42 



DESCRIPTION OF ROCK-MAKING MINERALS 119 

2. STATISTICAL METHOD. 

The method proposed by Michel Levy is practical 
in all sections, showing the albite twinning. This 
method consists in finding the maximum equal extinc- 
tions on opposite sides of an albite twinning line. The 
position of the plane which gives maximum extinctions 
in the zone normal to the brachypinacoid is different for 
different feldspars. This method, though tedious, is 
reliable, in that the various species have characteristic 
maxima. 

Sections normal to the brachypinacoid may be 
recognized by the fact that the twinned parts show equal 
illumination eight times upon a complete rotation of the 
stage, once every 45 degrees, in which position the 
two parts seem to belong to one individual. The faintly 
discernible twin line must be parallel to the plane of 
vibration of either of the nicols at equal illumination. 

Maximum extinction angles in sections perpendicular 
to albite twinning: 

Albite 16 

Oligoclase 2 

Labradorite 34 

Anorthite Over 37 

Monoclinic Feldspar. 

ORTHOCLASE. 
Composition : KAlSi s O 8 . 
Criteria for determination in thin section: 
See also under Feldspar group. 

Twinning after Carlsbad law common, after Baveno 
and Mannebach less common. 

Optical Properties : Biaxial and negative. 
Colorless in thin section. 

Refringence low. Relief absent and surface 
smooth. 



120 OPTICAL MINERALOGY AND PETROGRAPHY 

Birefringence very weak, with interference colors 

of the lower first order, bluish gray, white, etc., not 

quite as bright as the colors of quartz and plagioclase. 

Alteration to kaolinite so prevalent that surface 
usually appears cloudy. 

Differentiation: From other feldspars, see under 
Feldspar group. 

From quartz, by cloudy appearance, and negative 

character. 

Occurrence: Abundant in acid plutonic rocks, pres- 
ent in intermediate and certain basic igneous rocks, in 
schists, gneisses, and in contact zones. As perthite, with 
bands of albite. 

Uses: In the manufacture of porcelain and china. 
A variety of orthoclase called moonstone is used as 
a gem. 

SANADINE. 

Sanadine is a clear, glassy variety of orthoclase, 
occurring in rhyolite, trachyte, obsidian, etc. It decom- 
poses less readily than orthoclase, has a smaller axial 
angle, and usually contains more inclusions. 

Triclinic Feldspars. 

MICROCLINE. 
Composition: KAlSi 3 8 . 
General characters same as orthoclase. 
Differentiation: From orthoclase; simple crystals 
not showing the crossed twinning have extinction angles 
of about 15 degrees on the base with reference to the 
brachypinacoidal cleavage. 

From other feldspars by the characteristic crossed, 
rectangular, grating structure. 

Occurrence: Similar to orthoclase, but more abun- 
dant in pegmatites. 



DESCRIPTION OF ROCK-MAKING MINERALS 121 

ALBITE. 

Composition : NaAlSi 3 8 . 

Form and cleavage characteristic of Feldspar group. 

Optical Properties : Biaxial and positive. 
See under Feldspar group. 

Differentiation : From orthoclase, by the presence of 
polysynthetic twinning. 

From microcline, by the absence of grating 

structure. 

From other plagioclase feldspars, see under Feld- 
spar group. 

Occurrence: Seldom as a primary constituent of 
igneous rock except as an intergrowth with orthoclase 
or microcline in the form of perthite in soda-rich igneous 
rocks. 

OLIGOCLASE. 

Composition : Ab 6 An x to A^ An 2 . 

Differentiation : See under Feldspar group. 

Alteration: Kaolinization is less frequent than in 
albite. 

Occurrence: In eruptive rocks and in crystalline 
schists. More common in granites than albite is. 

LABRADORITE. 

Composition: AfyA^ to AbjAn.,. 

Differentiation: See under Feldspar group. 

Alteration : To a micaceous mass. To an aggregate 
composed of zoisite, epidote, albite, quartz, etc. 

Inclusions: Hematite and ilmenite abundant in col- 
ored varieties. 

Occurrence: Common in basic igneous rocks as 
gabbro, basalt, etc., with olivine, augite and mag- 
netite. Is the principal constituent of anorthosite. 
Occurs sparingly in meteorites. 

Uses : As an ornamental stone. 



122 OPTICAL MINERALOGY AND PETROGRAPHY 

ANORTHITE. 

Composition : CaAl 2 Si 2 O 8 . 

Differentiation: Anorthite has the strongest bire- 
fringence and the highest refringence of all of the feld- 
spars. See under Feldspar group. 

Occurrence : As an essential constituent of basic igne- 
ous rocks. 

Developed by contact and regional metamorphism. 



DESCRIPTION OF ROCK-MAKING MINERALS 123 



PART TWO. PETROGRAPHY. 

CHAPTER 8. 
General Discussion of Igneous Rocks. 

Petrography is that division of Petrology which is 
concerned with the systematic classification and descrip- 
tion of rocks megascopically and microscopically. 

The broad classification of rocks according to origin 
is: 1, Igneous; 2, Sedimentary, and 3, Metamorphic. 

Igneous rocks are those which have solidified by cool- 
ing from a molten condition. 

Sedimentary rocks are those which have been depos- 
ited under water or on land by mechanical, chemical or 
organic processes. 

Metamorphic rocks are those which are derived from 
previously existing igneous or sedimentary rocks by heat 
alone or by pressure and resultant heat. 

Igneous Rocks. 

Classification. Several methods for classifying igne- 
ous rocks have been devised, two of which seem to be 
more or less satisfactory for practical purposes. These 
methods are the qualitative and the quantitative classi- 
fications. By an examination of the minerals comprising 
the rock many inferences may be derived as to the mode 
of origin, the conditions of crystallization, the general 
chemical composition, whether acid or basic, etc. A 
macroscopic observation alone gives the observer some 
basis for a rough classification. In the field, the mining 



124 OPTICAL MINERALOGY AND PETROGRAPHY 

engineer or geologist may find a rock which is essentially 
quartz- and feldspar, with a small percentage of ferro- 
magnesian minerals. He may call it a granite. His clas- 
sification is correct if the rock contains 25 or 30 per 
cent of quartz. But if it contains only a few per cent, 
he will hesitate as to whether the rock is a granite or a 
syenite. 

As long as such a doubt exists as to the proper clas- 
sification of a rock, it is obvious that the system of 
classification is at fault. It is of course clear that all 
possible gradations in mineral percentages exist between 
the various igneous rock types. In so far as this is 
true, a qualitative classification based upon mineral per- 
centages is defective. On the other hand, this method 
is exceedingly rapid in that one who is skilled in the 
manipulation of the microscope and in the interpretation 
of the phenomena observed in thin section can infer much 
from a glance about the nature of the rock. 

For more complete descriptions of a rock, the quanti- 
tative classification is more satisfactory in that the chem- 
ical composition of the rock is used as a basis for clas- 
sification. But such an analysis usually takes two or three 
days of careful work by a skilled chemist. Obviously, a 
qualitative classification with the aid of the microscope 
meets the requirements of the great majority of cases 
generally met with. 

Essential and Accessory Minerals. Of the thousand 
minerals which are known, only about ninety occur in 
igneous rocks. Twenty-five of these are of prime impor- 
tance in determining the classification of a rock. These 
are the "essential minerals," for their presence is essen- 
tial to the classification and definition of the rock type in 
which they appear. The remaining minerals, which com- 
prise the majority, are the "accessory minerals," whose 



GENERAL DISCUSSION OF IGNEOUS ROCKS 125 

presence or absence does not influence the name under 
which the rock is classified. They are usually, though 
not always, present in small quantities. Typical acces- 
sory minerals are zircon, apatite, ilmenite, titanite, etc. 

It is to be noted that minerals which are not essen- 
tial to the definition of a large division, as the granite 
group, may become essential if the group is subdivided 
into a smaller division, as the amphibole-granite class. 

Primary and Secondary Minerals. Primary minerals 
are those which crystallized out from solution at the 
time of the solidification of the magma. Examples are the 
feldspars and quartz in granite. Secondary minerals are 
those which have formed after the solidification of the 
magma by the alteration of the previously existing min- 
erals, the alteration usually taking place through the 
agency of weathering. Examples are the alteration of 
the feldspars to kaolinite and muscovite, and of the 
amphiboles and the pyroxenes to serpentine. Secondary 
minerals are derived from primary minerals by the appli- 
cation of heat and pressure. Sericites are thus derived 
from impure quartzites; chlorite from amphiboles and 
pyroxenes; talc from amphiboles, pyroxenes and impure 
dolomites. 

Texture. Although composition is the chief means 
of distinguishing rock types according to the qualitative 
system, texture likewise plays an important role in that 
it furnishes an important clue as to the circumstances 
under which the rock was formed. There are thus two 
considerations to be taken into account in the identifi- 
cation of a rock the chemical composition, and the 
texture. 

Texture is defined as the size, shape and mode of 
aggregation of the constituent particles of a rock. It 
is determined by certain conditions prevailing in and 



126 OPTICAL MINERALOGY AND PETROGRAPHY 

about the molten magma at the time of the solidification 
of the rock mass. Most important of these are : 

1. The rate of cooling; 

2. The chemical composition of the magma; 

3. Pressure; 

4. Temperature; 

5. Action of mineralizers as steam, HC1, Fl, B. 

The rock solidifying at great depths cools very slowly, 
allowing the minerals time to crystallize into well-formed 
individuals. Many minerals crystallize simultaneously, 
and these minerals interfere with each other as they 
grow. The interpenetration or irregular boundary line 
between any two crystals is a mutual adjustment of 
simultaneous formation. Molten magmas which are 
suddenly subjected to rapid cooling, such as would accom- 
pany an extrusion on or near the surface, crystallize rel- 
atively rapidly, with the result that a portion of the rock 
mass crystallizes as a glass. Microscopic crystals usu- 
ally have time to make their appearance. It is also found 
that crystals of some minerals grow more rapidly than 
crystals of other minerals. When a rock shows a glassy 
appearance, with minute crystals embedded in the glass, 
the glass is regarded as a "groundmass." 

The common textures may be reduced to four, as 
follows : 

1. Glassy; 

2. Felsitic, or stony ; 

3. Porphyritic; 

4. Granitoid. 

Glassy texture is characterized by absence of 
crystallization. 

Felsitic or stony texture shows some crystallization of 
minute crystals enmeshed in a glassy or dense ground- 



GENERAL DISCUSSION OF IGNEOUS ROCKS 127 

mass, giving the rock a stony or noncrystalline 
appearance. 

Porphyritic texture results from conditions within 
the magma which allow the crystallization of certain min- 
erals to take place before any other appears. These well- 
defined crystals, which are called "phenocrysts," are 
embedded in a finer ground mass, which may be wholly 
glassy, partly crystalline, or very finely crystalline 
throughout. 

Granitoid texture is applied to those rocks which con- 
tain no groundmass and which are composed of crystals 
of the same general time of growth or which separated 
out in order of their basicity. In this case the earlier 
minerals show well-defined boundaries and crystal planes, 
whereas the later minerals fill the interstices and assume 
an irregular shape, determined by the position of the 
earlier minerals. 

Extrusive flows and intrusive lavas and dikes are 
usually characterized by the presence of a groundmass. 
The deep-seated rocks are characterized by a granitoid 
texture. All gradations in texture between rocks con- 
sisting entirely of glass and of wholly crystalline mate- 
rial exist. 

To illustrate the use of mineral composition and tex- 
ture in classifying rocks, the following examples are 
given : 

ROCK. MINERALS. TEXTURE- 

Granite Alkali feldspar and quartz . Granitoid. 
Syenite same . . . . Groundmass present. 

Diorite Acid feldspars . . . Granitoid. 
Andesite same . . . . Groundmass present. 

Gabbro - Lime feldspars . . . Granitoid. 
Basalt same . . . Groundmass present. 

Diabase same but with intermediate texture. 



128 OPTICAL MINERALOGY AND PETROGRAPHY 

Textural Terms. Convenient terms which are applied 
to igneous rocks to describe the amount of crystallized 
matter present, are : 

1. Glassy, in which no crystals are present ; 

2. Cryptocrystalline, in which crystals are present 
but visible neither to the eye nor to the microscope ; 

3. Microcrystalline, in which crystals are present but 
visible only under the microscope ; 

4. Hypocrystalline, in which the rock consists partly 
of glass and partly of crystallized matter ; 

5. Holocrystalline, in which the rock is completely 
crystallized and no glass exists. 

A classification of terms which describe the form and 
shape of the crystals : 

1. Idiomorphic crystals are those which have their 
own peculiar geometric form. The first minerals to crys- 
tallize from any solution are idiomorphic, as they were 
allowed to grow without interference. 

2. Hypidiomorphic crystals are those having part of 
their planes present and part absent. This may be 
brought about by an overlap in the time of crystallization 
of a series of minerals. One mineral is not given time 
to crystallize completely before an adjacent mineral 
interferes. 

3. Allotriomorphic crystals are those in which no 
crystallographic planes are present. This is true of the 
last minerals to crystallize. Their shape is determined 
by previously existing minerals. Simultaneous crystal- 
lization sometimes results in the development of allotrio- 
morphic crystals. 

Rosenbusch's Law. There is a normal order of crys- 
tallization in igneous rocks which in general is a law of 



GENERAL DISCUSSION OF IGNEOUS ROCKS 129 

decreasing basicity, and is determined by the presence 
or absence of silica, the chief acid radicle, in rock-form- 
ing minerals. It was first worked out by Rosenbusch 
(Heidelberg) and is briefly as follows: 

1. Minor accessories: Apatite, magnetite, hematite, 
ilmenite, pyrite, chalcopyrite, pyrrhotite, zircon, titanite, 
garnet. 

2. Ferro-magnesian minerals: Olivine, orthorhombic 
pyroxenes, monoclinic pyroxenes, amphiboles, biotite, 
muscovite. 

3. Feldspathic minerals: Plagioclase feldspars in 
order, from anorthite through bytownite, labradorite, 
andesine, oligoclase, to albite; orthoclase, feldspathoids, 
nephelite, leucite, sodalite. These latter minerals may 
crystallize out before or after the feldspars. 

4. Quartz, microcline. Quartz sometimes shows inter- 
growths with orthoclase. 

Volcanic and Plutonic Rocks. Igneous rocks are 
finally divided into two important types, which depend 
directly upon mode of occurrence. 

1. Volcanic or eruptive rocks are those which flow 
out upon the surface or are ejected into the upper crust 
of the earth near enough to the surface to assume a tex- 
ture characteristic of rapid cooling. Volcanic rocks have 
a glassy or porphyritic texture. 

They may be either holocrystalline porphyritic or 
hypocrystalline porphyritic. 

2. Plutonic or intrusive rocks are those which do not 
reach the surface except by subsequent erosion of the 
overlying strata. They take on a texture characteristic 
of slow cooling. Plutonic rocks are therefore holocrys- 
talline and granitoid, occasionally porphyritic. They 
may be distinguished from volcanic rocks by the absence 
of groundmass. 



130 OPTICAL MINERALOGY AND PETROGRAPHY 

Geological Occurrence. The following table lists the 
commonly observed original structures of igneous rocks. 

VOLCANIC ROCKS. 

Extrusive. 

1. Pyroclastic or f ragmen tal deposits, as ash or tuff. 

2. Volcanic necks. 

3. Lava flows or sheets. Overflow from fissures. 

VOLCANIC OR PLUTONIC ROCKS. 
Intrusive. 

4. Intrusive sheets or sills. 

5. Bysmaliths. 

6. Laccoliths. 

7. Dikes. 

PLUTONIC ROCKS. 
Intrusive. 

8. Bosses or stocks. 

9. Batholiths. 

Petrogeny. 

Magma. A magma is a fused rock mass in mutual 
solution. The essential feature of a solution is its ten- 
dency to become homogeneous. This tendency is pro- 
duced by diffusion, convection currents, differences in 
temperature, sinking of fragments of superincumbent 
rocks, etc. 

Magmas do not originate in the places where they are 
now observed. They move 

1. In the zone of flow : 

a. By rising gradually, like a bubble of air in wa- 
ter, with a flowage of the rocks above so as to allow 
passage ; 

b. By overhead stoping and absorption ; 

c. By assimilation. 



GENERAL DISCUSSION OF IGNEOUS ROCKS 131 

2. In the zone of fracture : 

a. By following the course of least resistance 
through whatever openings exist. 

b. By overhead stoping. 

Differentiation. The possible causes of differentia- 
tion in a still fluid magma are gravity and differences in 
temperature, of which gravity is by far the more impor- 
tant. This accounts for the accumulation and concen- 
tration of magnetite along the lower border of a magma. 
It is the first mineral to crystallize. 

Magmatic Stoping. Marginal assimilation is one of 
the methods of magma advance through overlying rock 
formations. This method is effective chiefly in the early 
part of the magma's history and takes place at the main 
contacts and along a relatively limited surface. 

According to the theory of magmatic stoping, each 
batholithic magma in its gradual advance upward 
through the overlying rocks engulfs large blocks of rock 
from the roof and walls. This process is facilitated by 
the shattering which it is believed accompanies an intru- 
sion, due to unequal heating of the country rock along 
the contacts. These blocks are thus dissolved at depths 
forming a compound magma by assimilation. The aver- 
age crust rock is more soluble in basic rocks than in acid. 

Crystallization. A eutectic is that proportion of two 
or more substances that has the lowest freezing point 
for those substances. Eutectic aggregates represent later 
products of crystallization because the first mineral to. 
crystallize is that which is in excess as compared with 
certain standard proportions. Thus, an intimate inter- 
growth of quartz and feldspar is a proof of simultaneous 
crystallization. 

If a third substance were added to a eutectic propor- 
tion, it would lower the temperature so as to approach 



132 OPTICAL MINERALOGY AND PETROGRAPHY 

a ternary eutectic, unless the third substance were pres- 
ent to an amount less than one per cent, in which case 
it may be considered negligible. A mineral which crys- 
tallizes late has an appreciable effect on a eutectic pro- 
portion. One which crystallizes early, as apatite, has no 
effect. Thus the importance of an accessory mineral 
depends upon its solubility in a eutectic, although it is 
usually present in such a small amount that it may be 
disregarded. 

On the other -hand, the gases will be present 
throughout the crystallization of the magma. They tend 
to lower the temperature of crystallization to a greater 
extent than do the accessory minerals, and they aid the 
magma to solidify to a crystalline mass instead of to a 
glass. 

In perfect isomorphism, A and B form mixed crystals 
in any proportion, so that there is a complete series of 
possible varieties between end members. Such a series 
is obtained between minerals which agree very nearly 
in molecular volume and crystalline elements. The albite- 
anorthite series is an example. 

In imperfect isomorphism only certain mixtures are 
possible, as A with some B, or B with some A. The ortho- 
clase-albite series is an example. Orthoclase may con- 
tain some albite, but no continuous series of mixtures 
connects pure orthoclase and pure albite. 

Influence of Gases on a Magma. Gases present in 
magmas are in a condition of unstable equilibrium, par- 
ticularly at slight depths, liberating heat by reaction with 
each other. With decrease in pressure the reaction 
between gases increases. Thus it was observed in 
Hawaii that the temperature of a lava lake changed a 
few hundred degrees in temperature as the amount of 
gases passing through it increased or decreased. The 



GENERAL DISCUSSION OF IGNEOUS ROCKS 133 

temperature increased with increase in the amount of 
gases. 

Relation Between Composition of Igneous Rocks and 
Magmas. A magma has a composition differing from 
that of an igneous rock by the amount of material in the 
magma which escapes prior to crystallization. No 
analyses are available showing the relative amounts of 
water vapor with other gases. Iddings believes that 
99.9% of all gases escaping from magmas consists of 
water vapor. Other gases are: CO,, N,, H,S, 0, HC1, 
H_,, SO,, S, CH 4 , Fl, B. 

The more basic the rock is the greater quantity of 
gases it contains. The liberation of all of the gases in 
the outer seventy miles of the earth would double the 
amount of nitrogen and carbon dioxide in the atmos- 
phere. Less than seventy miles of earth's crust during 
consolidation would yield all of the gases of the atmos- 
phere. Chamberlain believes that the gases of the atmos- 
phere have had that source. The same conclusion may 
be drawn with regard to the water of the hydrosphere, 
assuming that the average per cent of water in igneous 
rocks is 2.3. 

Aids in the Determination of Igneous Rocks in Hand 
Specimens. By a megascopic examination of an igneous 
rock it is sometimes possible to make a fairly good esti- 
mate as to what the rock is. Rocks with glassy or felsitic 
texture may easily be distinguished from granitoid or 
porphyritic rocks. By color it is possible to determine 
whether a rock is acid or basic, as the color is influenced 
by the amount of ferro-magnesian minerals. The rocks 
which are known to occur most commonly in nature 
should be given first consideration in examining the 
unknown rock. 



134 OPTICAL MINERALOGY AND PETROGRAPHY 

The texture of the rock should be examined first. Hav- 
ing noted the presence or absence of a groundmass, the 
feldspars should be examined. Pink feldspar is usually 
orthoclase or microcline. If Carlsbad twinning can be 
observed, the mineral is probably orthoclase or 
microcline. 

Plagioclase feldspars are usually white, gray or bluish 
gray, sometimes with a flashing blue surface. Albite 
twinning is often observed as a polysynthetic striation. 
Labradorite is of a darker blue or gray than the other 
feldspars. The feldspars are less transparent and glassy 
than quartz. 

Quartz is recognized by its vitreous, fresh appearance 
and transparent quality. A rock containing quartz will 
contain neither leucite nor nephelite. If leucite or nephe- 
lite can be determined, quartz is absent. This fact is 
inherent in the chemical composition of the magma. A 
rock containing leucite or nephelite is too low in silica 
for any to be present as quartz in excess. 

Dark minerals which appear in an orthoclase-micro- 
cline rock are biotite, hornblende, or augite. Biotite is 
determined by the flashing black surface due to the per- 
fect cleavage plane. A knife blade may be used to test 
the softness and the ease of cleavage. It is more diffi- 
cult to distinguish hornblende from augite in that they 
are both hard and not readily cleavable. In good crystals, 
augite shows an eight-sided cross section, whereas horn- 
blende has a six-sided cross section. 

The black minerals of a plagioclase rock are biotite 
and hornblende rather than augite. If the rock is mainly 
basic, has a dark color, and a dull, stony appearance, the 
black mineral is probably augite. Olivine occurs in basic 
lavas in clear, glassy greenish-yellow grains. 



GENERAL DISCUSSION OF IGNEOUS ROCKS 135 

A dense, volcanic rock which shows a groundmass and 
visible quartz is either rhyolite or dacite. Without fur- 
ther examination the observer would be justified to call 
the rock rhyolite, as it is far more abundant than dacite. 

If the volcanic rock is black and felsitic or stony in 
appearance, it is a basalt. If the rock answers neither 
of these descriptions but is evidently volcanic, it may be 
a trachyte, a phonolite, or an andesite. Of the three, 
andesite is the most probable, as it is the most common. 
It usually appears medium dark, midway between the 
acid and basic members of the series. If Carlsbad twin- 
ning is seen on the feldspar, the rock may be trachyte 
instead of rhyolite. If leucite is distinguished, it is a 
phonolite, otherwise there would be no justification for 
naming it thus. 

A rock possessing a granitoid texture and quartz in 
some abundance may be called a granite rather than the 
rarer quartz diorite. If it contains orthoclase and no 
quartz, the observer would doubtless classify the rock 
either as a syenite or as a nephelite syenite, although the 
former would be the more probable. Diorites are darker 
than the syenites and may be inferred from this fact 
alone if the character of the feldspars cannot be deter- 
mined. If plagioclase can be distinguished the classifi- 
cation is simplified. The latter may also show the dark 
or gray blue color of labradorite. 

Diabase may be determined by a peculiar texture, 
commonly called diabasic texture, on account of its char- 
acteristic appearance. White plagioclase is intimately 
intergrown with augite crystals, the plagioclase having 
developed first, hence taking a lath-shaped texture. The 
interstices between the feldspars were later filled by the 
augite. It is dark and of medium grain. 



136 OPTICAL MINERALOGY AND PETROGRAPHY 

The more basic rocks are determined by the total 
absence of quartz and feldspar, and the nature of the 
ferro-magnesian mineral comprising the greater part of 
the rock. 



GENERAL DISCUSSION OF IGNEOUS ROCKS 



137 



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138 OPTICAL MINERALOGY AND PETROGRAPHY 



CHAPTER 9. 

IGNEOUS ROCK TYPES. 

Plutonic Rocks. 
THE GRANITE FAMILY. 

Mineralogical Composition. Essential minerals: al- 
kali feldspar and quartz. Common minerals: biotite, 
muscovite, amphiboles, pyroxenes. Accessory minerals : 
magnetite, apatite, zircon, titanite, garnet, tourmaline. 

Texture. Granitoid. 

Character. Granites are generally light in color, in 
shades of white, gray and pink, occasionally darker, due 
to an increasing amount of biotite, amphiboles or pyrox- 
enes, in which case the rocks are liable to grade into the 
syenite or diorite families. 

Microcline is more common in granites than in any 
other kind of a rock. The quartz may contain minute 
rutile needles or tiny cavities filled with gas bubbles. Bio- 
tite is the commonest dark silicate. 

Varieties of Granites. There are two varieties of 
granites. The most common type consists of the alkali- 
lime variety, and the rarer type is called the alkali-granite 
variety. The essential difference between the two types 
is that the alkali-lime variety grades toward and into the 
diorite family, and the alkali-granite variety grades 
toward and into the alkali syenites and finally into the 
nephelite syenites. The pyroxenes and amphiboles of the 
alkali-lime variety are more basic, and contain consider- 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 139 

able amounts of magnesium and calcium. These min- 
erals do not appear in the alkali granites, but are substi- 
tuted by alkali pyroxenes and amphiboles, such as segi- 
rite and riebeckite. 
Classification : 

GRANITE FAMILY. 

Alkali-Lime Granites. Containing alkali feldspar 
(orthoclase, microcline, albite, perthite) and quartz. 

a. Granitite, with addition of biotite. 

b. Amphibole granitite, with addition of biotite and 
amphibole. 

c. Pyroxene granitite, with addition of biotite and 
pyroxene. 

d. Granite, with addition of muscovite and biotite. 

e. Amphibole granite, with addition of muscovite, bio- 
tite and pyroxene. 

f. Pyroxene granite, with addition of muscovite, bio- 
tite and pyroxene. 

g. Tourmaline granite, with addition of tourmaline. 

Alkali Granites. Containing alkali feldspar and 
quartz. 

a. Alkali granitite, with addition of biotite. 

b. Aegirite granite, with addition of segirite. 

c. Riebeckite granite, with addition of riebeckite. 

d. Aplite, no subordinate mineral except possibly a 
little muscovite. 

BORDER PHASES OF GRANITES. 

Pegmatites. A pegmatite is a border phase of a gran- 
ite often observed on the edges of bosses and batholiths. 
They are usually very coarsely crystalline vein-granites, 
consisting of quartz, feldspar, muscovite, tourmaline, 
beryl, spodumene and others. Due to the immense size 



140 OPTICAL MINERALOGY AND PETROGRAPHY 

which the crystals attain, pegmatites are sometimes 
called "giant granites." The largest crystal of spod- 
umene on record was found in the Etta tin mines of the 
Black Hills. This crystal measured thirty feet in height. 
Beryl crystals weighing over a ton have been recorded. 
Muscovite mica in sheets three feet in diameter are quite 
common. In pegmatites, the essential minerals of gran- 
ites are not always present. Quartz and beryl, quartz and 
tourmaline, mica and quartz, feldspar and tourmaline, 
are all possible combinations. 

Pegmatites are usually regarded as a late phase of 
the eruption which produced the granite. The common 
occurrence of such minerals as tourmaline, top'az and 
apatite in pegmatite leads to the suggestion that the influ- 
ence of the rare elements fluorine and boron may have 
had some influence in effecting the coarse crystallization 
which so frequently exists. 

Graphic Granite. Graphic granite is a variety of a 
pegmatite which consists of a curious form of inter- 
growth of quartz and the feldspars in such a manner that 
the cross fracture of the vein rock exposes a cuneiform 
or wedge-shaped texture resembling the writing charac- 
ter of the ancient Chaldeans and Assyrians. The most 
common intergrowth is quartz inclosed in orthoclase, 
microcline, or perthite. Since neither of the minerals 
comprising graphic granite possesses any definite crystal 
shape, it is evident that they crystallized from solution 
at the same time. 

Greisen. Greisen is another border phase of a gran- 
ite mass which, although not occurring abundantly, is of 
economic value the world over as the mother rock for 
cassiterite, the tin ore. It is a granitoid rock composed 
of quartz and muscovite, or some related white mica, as 
lepidolite or zinnwaldite. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 141 

Greisens are the result of contact action on gran- 
ites under the influence of mineralizers. It has been 
suggested that the water and fluorine vapors came into 
contact with the feldspars, converting them to micas. 

Orbicular Granite. Orbicular granite consists of 
spherical or ellipsoidal masses of basic minerals in gran- 
ite. The dark minerals are commonly biotite, pyroxenes, 
or amphiboles. They exist in the nature of basic segre- 
gations. 

Contact Metamorphism. Owing to the amount of 
heat and the presence of mineralizers which are given off 
from the granitic magma, considerable metamorphic 
effect is exerted upon adjacent rocks with, which the 
intruding magma comes in contact. This effect may not 
be conspicuous upon igneous and previously metamor- 
phosed rocks, as they are already in a highly crystalline 
condition. The effect upon sedimentary rocks is usually 
considerable. It takes the form of a more or less com- 
plete recrystallization of the secondary and undecom- 
posed primary minerals composing the rocks. In addi- 
tion to complete recrystallization there is quite frequently 
an addition or a subtraction of constituents or an ex- 
change of constituents with the magma. 

The chief effect upon sandstone is a recrystallization 
of the rounded grains of sand, resulting in a filling of the 
interstices between the grains. Quartzite is the rock de- 
veloped. If the sandstone contained considerable clay and 
other impurities, a sericite schist would develop. 

A common effect of an intrusive granite in contact 
with a limestone formation is the recrystallization of the 
limestone to a marble. Most limestones contain consider- 
able percentages of argillaceous and siliceous material, 
which will be converted to lime magnesium silicates. If 
there is no opportunity for the carbon dioxide of the 



142 OPTICAL MINERALOGY AND PETROGRAPHY 

limestone to escape, it will be retained in the marble as 
calcite or dolomite. If there is an opportunity for its 
release through fissures or joint cracks, the resulting 
mass may consist essentially of secondary silicates, some 
of which develop by a recrystallization of the original 
constituents of the limestones and others by the addition 
of material from the magma. 

The contact metamorphic effect of an intrusive magma 
on shale is pronounced and characteristic. Immediately 
at the contact, the shale is converted into a "hornfels" 
rock, which is dense, very finely crystalline, extremely 
hard, has a conchoidal fracture and consists chiefly of 
quartz, feldspar and biotite. From hornfels to the unal- 
tered shale the following stages are often observed: 
highly metamorphosed mica schist, spotted mica slate, 
spotted slate lacking the conspicuous development of the 
micas, unaltered shale. This change may be almost im- 
perceptible, and may extend for miles from the actual 
contact. The chemical composition of hornfels and shale 
are often very similar, although at times it shows an addi- 
tion of material in the hornfels. 

Economic Uses of Granites. Granite is more exten- 
sively used for structural purposes than any other igne- 
ous rock, although any crystalline rock is often loosely 
called granite in the quarry if it consists of silicates. 
Granite is the strongest of the common building stones, 
the crushing resistance ranging from 15,000 to 30,000 
pounds per square inch tested on two-inch cubes. 

The following resistance tests show the average in 
range : 

Granite from St. Cloud, Minn., 26,250 to 28,000. 

Granite from Mystic River, Conn., 18,125 to 22,250. 

Granite from Cape Ann, Mass., 19,500. 

Granite from Vinal Haven, Maine, 25,700. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 143 

Upon the following points are based the desirability 
of granites for structural purposes : 

1. Homogeneity of texture. 

2. Adaptability to tool treatment. 

3. Good rectangular jointing in the quarry. 

4. Pleasing color. 

5. Transportation facilities. 

6. Durability as affected by grain and mineral- 
content. 

A light color is generally more desirable than a dark 
one, and a medium grain is more favorable for durability 
than a coarse grain. The Rapakiwi granite of southern 
Finland is used freely in Petrograd for columns. It con- 
tains large red orthoclase crystals, which give the rock 
a prevailing red color, greenish plagioclase, smoky quartz 
and biotite. The disintegration is found to be rapid, as 
the jointing or fracture occasioned by the cleavage planes 
of one mineral tends to continue into the others. 

Relationship. Granite approaches syenite by insensi- 
ble gradations with decrease in quartz. It approaches 
diorite with increase in hornblende or biotite and plagio- 
clase. Intermediate varieties are called granodiorites. 
With increase in augite and plagioclase, granite ap- 
proaches gabbro. 

Geographical Distribution. Granite occurs abun- 
dantly along the Atlantic Coast from Virginia into Can- 
ada. It is extensively quarried. The Quincy granite 
from Quincy, Mass., is a well-known building granite. 
In Minnesota, Wisconsin, and northern Michigan, and 
northward in Ontario, much of the Pre-Cambrian crys- 
talline area known geologically as the Laurentian High- 
land is granite. It is found widespread in the West, 
existing in the Black Hills, in the Wasatch, the Rocky 
and the Sierra Mountains. 



1 


2 


3 


4 


5 


6 


7 


73.23 


77.50 


69.00 


67.70 


61.90 


69.46 


66.84 


. 15.47 


10.10 


14.80 


14.80 


13.20 


17.50 


18.32 




.... 


2.30 


2.10 


3.60 


2.30 


2.27 


. 3.34 


2.70 


.90 


3.40 


2.30 


.... 


.20 


.24 


.60 


1.10 


1.60 


4.60 


.30 


.81 


. .80 


2.30 


3.80 


3.90 


3.50 


2.70 


3.31 


1.70 


3.20 


2.50 


4.10 


2.70 


2.93 


5.14 


. 4.38 


4.00 


4.50 


4.30 


6.10 


4.07 


2.80 


.65 


.3 


.7 


1.00 


1.10 


.82 


.46 


. 99.81 


100.70 


99.60 


102.00 


99.00 


99.95 


100.49 






2.68 


2.62 


2.72 


2.68 


.... 



144 OPTICAL MINERALOGY AND PETROGRAPHY 

'V 

Analyses of Granites : 

SiO 2 . 
A1 2 3 
Fe,0, 
FeO 
MgO . 
CaO 
Na 2 O . 
K 2 O 
H,O . 

Total 
Sp. Gr. 

1. Granite from Carlsbad, Bohemia, Austria. 

2. Granite from Baveno, Lake Maggiore, Italy. 

3. Granite from Barr, Lower Alsace, Germany. 

4. Amphibole granitite from Barthoga, Sweden. 

5. Pyroxene granite from Lavellme, Vosges Mountains, France. 

6. Alkali granite from Chester, Massachusetts. 

7. Augite soda granite from Kekequabic Lake, Minnesota. 

Discussion of Analyses: 

1. Granite contains more silica than any other plu- 
tonic rock. 

2. Alumina content is not as high as in the syenites. 
It is present chiefly in feldspars and biotite. 

3. Iron content is generally low. It is present chiefly 
in biotite, amphiboles, pyroxenes, and possibly magnetite. 

4. Magnesia content is low, indicating an absence of 
many ferro-magnesian minerals. 

5. Lime content is low. It is present chiefly in a 
few acid plagioclases, in amphiboles and pyroxenes. 

6. Potash predominates over soda, occurring in 
alkali feldspar and biotite. 

7. Soda rarely predominates over potash. When it 
does (see Analysis 7), albite is the chief feldspar. It 
marks a gradation toward the diorites. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 145 

8. A high water content is probably due to the 
formation of secondary minerals, as serpentine, kaolinite, 
etc. Water in small quantities exists in many primary 
minerals: micas, amphiboles. 

9. The presence of apatite as an accessory mineral 
is indicated by the presence of P 2 0-,. 

10. The darker granites have the higher specific 
gravities. 

11. Granite is similar to rhyolite in composition. 

12. The alkali-lime granites contain more iron, mag- 
nesia, and lime than do the alkali granites. The alkali 
granites are richer in soda, potash, and possibly silica. 

THE SYENITE FAMILY. 

Mineralogical Composition. Alkali feldspar with 
little or no quartz. 

Texture. Granitoid with groundmass absent. Some- 
times porphyritic. 

Character and Distribution. Syenites are allied with 
nephelite syenites, into which they grade with increase 
of soda. They merge into the diorites with increase in 
plagioclase. Their geological occurrence is practically 
the same as that of granites. They are often found at 
the rims of granite bosses or batholiths where there has 
been a decrease in silica in the form of quartz. 

Geographically the syenites form the basement of 
the White Mountains, and occur in dikes near Little 
Rock, Arkansas. Many minor occurrences have been 
recorded. VARIETIES. 

Alkali-Lime Syenite. Essential minerals are alkali 
feldspar, with a little basic plagioclase. 

a. Amphibole syenite, with addition of amphibole. 

b. Mica syenite, with addition of mica. 

c. Pyroxene syenite, with addition of pyroxene. 



146 



OPTICAL MINERALOGY AND PETROGRAPHY 



Alkali Syenite. The alkali syenites are rare. The 
same three types occur in this group as occur in the alkali- 
lime group, except that the dark minerals are alkali 
pyroxenes and amphiboles. 

Belonging to the alkali-lime group is a rock called a 
"monzonite," which grades over into the diorites, as it 
contains both alkali feldspar and plagioclase. A rock 
associated with the copper ore deposits of Butte, Mon- 
tana, is a more acid rock of this type, called a quartz 
monzonite. It covers an area seventy miles by forty, and 
is known as the Bowlder Batholith. 

The corundum syenites north of Kingston, Ontario, 
are alkali syenites composed of pink orthoclase. and a 
greenish corundum which is used as an abrasive. 

The colors of the syenites are light, although usually 
darker than the granites. The crushing strength is 
greater. 

Analyses of Syenites: 

Si0 2 . 
TiO 2 . 

Fe 2 O 3 . 
FeO . 
MgO . 
CaO . 
Na : . 

H 2 
P 2 S . 

Total . 
Sp. Gr. 

1. Mica syenite (alkali type) from Tonsenoos, near Christiania, 

Norway. 

2. Mica syenite (alkali lime type) from Gangenbach, Black 

Forest, Germany. 



1 


2 


3 


4 


5 


6 


64.00 


51.00 


59.40 


52.88 


59.78 


59.83 




1.80 


.30 








17.40 


14.50 


17.90 


20.30 


16.86 


16.85 


. 1.00 


4.20 


2.00 


3.63 


3.08 





2.30 


4.40 


6.80 


2.58 


3.72 


7.01 


. .60 


8.20 


1.80 


.79 


.69 


2.61 


1.00 


5.10 


4.20 


3.03 


2.96 


4.43 


. 6.70 


1.80 


1.20 


5.73 


5.39 


2.44 


6.10 


7.20 


6.70 


4.50 


5.01 


6.57 


. 1.20 


1.00 


.40 


1.01 


1.58 


1.29 




.70 


.60 


.54 






101.40 


99.90 


101.30 


100.99 


99.07 


101.03 







2.77 


2.73 


2.67 


2.73 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 147 

3. Hornblende syenite from Biella, Piedmont, Italy. 

4. Augite syenite from Byskoven, near Laurvik, Norway. 

5. Syenite from Custer County, Colorado. 

6. Hornblende syenite from Plauenschen Grund, near Dresden. 

Discussion of Analyses: 

1. Silica content is lower than in granites, due to de- 
crease of quartz. 

2. Alumina content is higher than in granites, due to 
relative increase in feldspars. 

3. Iron, magnesia and lime contents are higher, due 
to increase in ferro-magnesian minerals, chiefly horn- 
blende. 

4. Alkali content is higher, due to increase of felds- 
spars. 

5. The high water content is due to hydration of the 
secondary minerals. 

6. The specific gravity is higher in that of granites, 
due to the increase in ferro-magnesian minerals. 

7. The alkali-lime syenites contain more lime and 
magnesia than the alkali syenites. 

NEPHELITE AND LEUCITE SYENITES. 

Mineralogical Composition. Alkali feldspar and 
nephelite or leucite. 

Texture. Granitoid, sometimes porphyritic. 

Character and Distribution. Nephelite and leucite 
syenites are white to smoky gray in color, and contain 
very few accessory minerals. When present, they usually 
are biotite, segirite, and an alkali amphibole called bar- 
kevikite. 

These types are comparatively rare, occurring espe- 
cially as dikes. They are known in North America at 



148 OPTICAL MINERALOGY AND PETROGRAPHY 

Montreal and Dungammon (Ontario), Litchfield 
(Maine), Red Hill (New Hampshire), Salem (Massa- 
chusetts), Beemersville (New Jersey), and near Little 
Rock (Arkansas), in well-known exposures, though they 
have a widespread occurrence. 

Of economic importance is the occurrence of rare min- 
eral containing zirconium, tantalum, titanium, yttrium, 
cerium, lanthanum, terbium, and other rare elements. In 
the nephelite-syenite pegmatites of southern Norway 
about 800 of these rare minerals have been recorded. A 
corundiferous nephelite syenite is found in commercial 
quantities in Canada. 

Analyses of Nephelite and Leucite Syenites: 





1 


2 


3 


4 


SiO, . 


56.30 


50.36 


60.39 


50.90 


AUO, 


. 24.14 


19.34 


22.51 


19.67 


Fe 2 O, . 


1.99 


6.94 


.42 


7.76 


FeO 






2.26 




MgO . 


.13 


.... 


.13 


.36 


CaO 


69 


3.43 


.32 


4.38 


Na,O . 


9.28 


7.64 


8.44 


4.38 


K 2 


. . . 6.79 


7.17 


4.77 


6.77 


H 2 O . 


1.58 


3.51 


.57 


1.38 



Total 100.90 98.39 99.81 100.01 

1. Nephelite syenite from Ditro, Transylvania, Hungary. 

2. Nephelite syenite from Beemersville, Sussex County, New 

Jersey. 

3. Nephelite syenite from Litchfield, Maine. 

4. Leucite syenite from Magnet Cove, Arkansas. 

Discussion of Analyses: 

1. Silica content is lower than in the syenites, as neph- 
elite has 44%. silica, and the minerals which it replaces 
have several per cent more. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 149 

2. Alumina content is higher than in any other plu- 
tonic rock, due to the presence of nephelite or leucite. 

3. Iron content is variable, but magnesia and lime 
contents are lower than in the syenites, due to the absence 
of many ferro-magnesian minerals. 

4. Alkali content is higher than in any other plutonic 
rock. Soda predominates over potash in the nephelite 
syenites. In the leucite syenites potash increases, but 
may not predominate, as leucite readily decomposes, 
allowing the potash to be removed. 

5. The specific gravity is less than that of the syenites. 

DIORITE FAMILY. 

Mineralogical Composition. Acid plagioclase with or 
without quartz, and some dark mineral, most commonly 
an amphibole near green hornblende. Biotite is common. 

Texture. Granitoid, at times porphyritic. 

Character. The color of diorite is dark, due to the 
ferro-magnesian minerals. It grades from the alkali-lime 
granite type by decrease in alkali feldspar and increase 
in acid plagioclase. Certain intermediate phases are 
called granodiorites, containing both alkali feldspar and 
acid plagioclase. 

Diorites are not very common in North America. 
They occasionally form on the edge of granite bosses or 
batholiths. 

CLASSIFICATION OF THE DIORITES. 
With Quartz Without Quartz 

Quartz mica diorite. Mica diorite. 

Quartz hornblende diorite. Hornblende diorite. 
Quartz augite diorite. Augite diorite. 

Quartz hypersthene diorite. Hypersthene diorite. 



150 



OPTICAL MINERALOGY AND PETROGRAPHY 



Analyses of Diorites: 




1 


2 


3 


4 


5 


Si0 2 


61.22 


64.12 


56.09 


52.45 


52.00 


A1 2 3 


. 16.14 


16.50 


16.03 


18.63 


15.75 


5'e 2 0. 


3.01 


2.71 


3.12 


11.40 


3.55 


FeO . 


. 2.58 


4.26 


4.77 


1.19 


12.84 


MgO . 


4.21 


2.34 


8.03 


5.16 


3.42 


CaO . 


. 5.46 


4.76 


6.73 


6.84 


7.39 


NajO . 


4.48 


3.92 


3.49 


2.64 


2.37 


K 2 O . 


. 1.87 


1.92 


1.87 


.37 


1.24 


H 2 


.44 


.73 


.16 


2.40 


.35 



Total 



99.41 101.26 



100.13 100.82 99.91 
Electric Peak, Yellow- 



1. Pyroxene amphibole biotite diorite. 

stone Park. 

2. Quartz mica hypersthene diorite from Pfundersberg, Tyrol. 

3. Mica hypersthene diorite from Campomaior, Portugal. 

4. Amphibole diorite from Neunseestein Barr, Alsace. 

5. Augite diorite from Richmond, Minnesota. 

Discussion of Analyses: 

These analyses when compared with the analyses of 
granite show that 

1. Iron, lime, magnesia and alumina contents are 
higher. 

2. Alkali content is lower. 

3. Soda predominates over potash. 

4. Silica is lower, due to change of feldspar. 

5. The specific gravity is higher. 

GABBRO AND NORITE FAMILY. 

Mineralogical Composition. Basic plagioclase and 
usually a pyroxene. 

Texture. Granitoid. Never porphyritic. 
Relationship. Gabbros grade by decrease in plagio- 
clase to the more basic pyroxenites and peridotites. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 151 

Varieties. Essential to all, basic plagioclase. 

1. Gabbro, with addition of diallage. 

2. Hornblende gabbro, with addition of hornblende. 

3. Olivine gabbro, with addition of olivine and 
diallage. 

4. Norite, with addition of hypersthene, bronzite or 
enstatite. 

5. Olivine norite, with addition of hypersthene, bron- 
zite or enstatite and olivine. 

6. Anorthosite, composed chiefly of labradorite. It 
may contain a few dark minerals which, when metamor- 
phosed, cause the development of almandite garnets in 
considerable quantities. 

Concentration of Magnetite. The concentration of 
magnetite in many gabbroid magmas took place during 
the process of solidification along the lower border of 
the magma. This concentration was effected by the early 
crystallization of the magnetite from solution, its high 
specific gravity, convection currents, etc. Magnetite of 
this occurrence has been found in commercial quantities 
in the Adirondacks and in Lake and Cook counties of 
northern Minnesota. It is usually titaniferous, due to 
an intimate association with the mineral ilmenite. 

Nickeliferous Pyrrhotite. In the Sudbury district of 
Ontario, great quantities of nickeliferous pyrrhotite and 
workable amounts of chalcopyrite are found in the norite. 
They occur as magmatic segregations. The pyrrhotite 
is an important source of nickel. 

In Lancaster County, Pennsylvania, nickeliferous 
pyrrhotite is observed along the contact of a metamor- 
phosed basic igneous rock called amphibolite. 

Platinum is believed to occur minutely disseminated 
in rocks of this type, the weathering of which has sftp- 
plied the placer deposits. 



152 



OPTICAL MINERALOGY AND PETROGRAPHY 



Analyses of Gabbros and Norites: 



SiO,, 

AW, 

Fe 2 O ; 

FeO 

MgO 

CaO 



1 


2 


3 


4 


5 


54.47 


44.10 


46.70 


49.10 


49.95 


26.45 


24.50 


22.20 


21.90 


19.17 


1.30 


7.90 


.80 


6.60 


4.72 


.67 


6.50 


5.50 


4.50 


6.71 


.69 


3.80 


10.30 


3.00 


5.03 


10.80 


12.00 


11.70 


8.20 


9.61 


4.37 


1.70 


1.70 


3.80 


3.13 


.92 


.20 


.10 


1.60 


.74 


.53 


.60 


1.10 


1.90 


.09 



K 2 O 
H 2 O 



Total . . . 100.20 101.30 100.10 100.89 99.84 

Sp. Gr. . . . 2.72 3.04 3.02 2.94 2.94 

1. Anorthosite from Adirondacks, New York. 

2. Gabbro from Mount Hope, Baltimore, Maryland. 

3. Olivine gabbro from Langenlois, Lower Austria. 

4. Hornblende gabbro from Duluth, Minnesota. 

5. Norite from Mpnsino, near lorea, Piedmont, Italy. 

Discussion of Analyses. The analyses compared with 
analyses of diorites show : 

1. A lower silica content due to the absence of quartz 
and the decreasing basicity of the feldspars. 

2. Higher alumina, lime, iron and magnesia content. 
High magnesia, as in Analysis 4, suggests olivine. 

3. Lower alkali content. 

4. Higher specific gravity. 

ESSEXITE FAMILY. 

Mineralogical Composition. Basic plagioclase, with 
varying amounts of subordinate orthoclase. Nephelite 
or sodalite may be present. The dark minerals are au- 
gite, biotite, and a brown amphibole called barkevikite. 
Olivine and apatite sometimes occur. The plagioclase 
isusually labradorite, rarely andesine. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 153 

Relationship. Essexite is related to the gabbros much 
as monzonite is related to the syenites. It was originally 
classed with the gabbros, but is more generally associated 
with the alkali and nephelite syenites. It may be con- 
sidered intermediate between this type and the gabbroid 
type. It was first recognized in association with neph- 
elite syenites near Boston. 

Discussion of Analyses (See next table) : 

1. Low silica content. 

2. High alumina and iron content. 

3. Equal amounts of lime and the alkalies. 

4. Soda predominates over potash. 

5. Magnesia content low as compared with gabbro. 

6. P 2 O 5 high due to apatite. 

THERALITE-SHONKINITE-MALIGNITE FAMILY. 
Mineralogical Composition. Basic plagioclase, neph- 
elite or leucite, pyroxene with some biotite, and rarely 
amphibole. Members of the sodalite group may accom- 
pany the nephelite. 

Relationship. These rocks are related to essexite, 
and grade into them. The presence of nephelite or leu- 
cite is the essential difference. Chemically they are quite 
similar. 

Distribution. Theralite was first found in the Crazy 
Mountains, Montana, near Livingston, and described by 
J. E. Wood, of Harvard. 

Shonkinite was described by Weed and Pirrson, from 
Square Butte in the Highwood Mountains of Montana, as 
a border phase of a sodalite syenite laccolith. It contains 
little nephelite, but has instead sanadine. Consequently, 
potash prediminates over soda. In theralite, soda 
predominates. 



154 OPTICAL MINERALOGY AND PETROGRAPHY 

Malignite was named by Lawson, from Puba Lake, 
Ontario. It contains chiefly segirite, augite, biotite, ortho- 
clase, nephelite and titanite. 

Discussion of Analyses: 

1. Chemical resemblance to essexite. 

2. Low silica. 

3. Equal lime and magnesia content. 

4. Magnesia higher than in essexite. 



Analyses : 



SiO 2 
A1 2 (X 



FeO 

MgO 

CaO 

Na 2 

K 2 

H 2 



1 


2 


3 


4 


5 


6 


47.94 


43.17 


46.73 


47.85 


42.79 


46.06 


17.44 


15.24 


10.05 


13.24 


21.59 


10.74 


6.84 


7.61 


3.53 


2.74 


4.39 


3.17 


6.51 


2.67 


8.20 


2.65 


2.33 


5.61 


2.02 


5.81 


4.68 


5.68 


1.87 


14.74 


7.47 


10.63 


13.22 


14.36 


11.76 


10.55 


5.63 


5.68 


1.81 


3.72 


9.31. 


1.31 


2.79 


4.07 


3.76 


5.25 


1.67 


5.14 


2.04 


3.57 


1.24 


2.74 


.99 


1.44 


1.04 


.... 


1.51 


2.42 


1.70 


.21 



Total . . 99.02 98.45 99.73 100.65 98.40 98.97 

1. Essexite from Salem Rock, near Boston. 

2. Theralite from Martinsdale, Crazy Mountains, Montana. 

3. Shonkinite from Square Butte, Highwood Mountains, Mon- 

tana. 

4. Malignite from Puba Lake, Rainy Lake District, Ontario. 

5. Ijolite from Iwaara, Finland. 

6. Missourite from Shonkin Creek, Highwood Mountains, Mon- 

tana. 

IJOLITE AND MISSOURITE. 

Mineralogical Composition. Ijolite contains aegirite- 
augite and nephelite, often with apatite, titanite, and 
andradite as accessories. It is nonfeldspathic. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 155 

Missourite contains augite, leucite, olivine, and biotite 
with accessories. It is nonfeldspathic. 

Relationship. These rocks are end products of the 
series beginning with essexite. They are closely related 
to the theralite-shonkinite rocks and are distinguished 
from them by the fact that they contain no feldspars. 
Ijolite was first found on Mount Iwaara in northern 
Finland. 

PERIDOTITE FAMILY. 

Mineralogical Composition. Olivine with pyroxenes, 
amphiboles, or biotite. No feldspars are present. 

Relationship. The peridotites grade from olivine 
gabbros by the elimination of the feldspars. They are 
found on the edges of gabbro and norite bosses. They 
are regarded as ultra basic. 

Classification. Olivine is essential in all varieties. 

1. Sherzolite, by addition of diopside and enstatite. 

2. Harburgite, by addition of enstatite. 

3. Wehrlite, by addition of diallage and hornblende. 

4. Cortlandite by addition of hornblende. 

5. Dunite chiefly olivine. It may contain chromite or 
chrome spinel. 

6. Kimberlite, which was named from its occurrence 
in Kimberley, South Africa, is a peridotite found in the 
truncated cones of extinct volcanoes. In its type local- 
ity it weathers to a soft serpentine rock called "blue 
ground." It is the mother rock of the diamond. A sim- 
ilar rock has been found in southern Arkansas, where 
diamonds are likewise found in commercial quantities. 
Small diamonds have been found in a peridotite rock in 
Elliot County, Kentucky. 

Garnierite, the chief ore of nickel, is a secondary min- 
eral associated with serpentinized peridotite, probably as 



156 



OPTICAL MINERALOGY AND PETROGRAPHY 



an alteration of a nickel-bearing olivine. The French 
locality of New Caledonia is the only important locality. 

Analyses : 





1 


2 


3 


4 


SiO. 


. . 41.44 


34.98 


53.98 


44.01 


ALOs 


. 6.63 


10.80 


1.32 


11.76 


FeoOt 


13.87 


1.42 


1.41 


15.01 


FeO 


. 6.30 


21.33 


3.90 




MgO . 


18.42 


19.30 


22.59 


25.25 


CaO 


. 7.20 


.43 


15.49 


4.06 


Na 2 . 


.24 


.17 




.... 


K 2 


. . .93 


5.42 






H 2 . 


5.60 


1.28 


.83 






100.63 



95.13 
3.276 



99.59 
3.301 



100.09 



Total .... 
Sp. Gr. 

1. Amphibole peridotite from Sebreizheim, Baden, Germany. 

2. Mica peridotite from Kaltes Thai, Harzburg, Germany. 

3. Pyroxenite from Baltimore, Maryland. 

4. Pyroxenite from Meadowbrook, Montana. 

Discussion of Analyses: 

1. Lower silica and alumina content than in gabbro, 
due to the absence of feldspars. 

2. Iron content varies, depending upon the dark min- 
eral present. 

3. Magnesia content higher than in any other normal 
plutonic rock. 

4. Lime content varies. 

5. Alkali content less than that of any other igneous 
rock. In a mica peridotite, potash predominates over 
soda, an unusual case among alkali-lime rocks. 

6. Specific gravity highest of the normal plutonic 
rocks. 



IGNEOUS ROCK TYPES PLUTONIC ROCKS 157 

PYROXENITE AND HORNBLENDITE FAMILY. 

Mineralogical Composition. These rocks consist of a 
single pyroxene, or a single amphibole, or two or more 
minerals of the same group. 

Relationship. They are the end products of the ultra 
basic rocks grading from the peridotites by the elimina- 
tion of the olivine. The varieties depend upon the min- 
eral which composes the rock, the rock name usually 
being the mineral name with the suffix "ite" added to it. 

Varieties of the family are diallagite, enstatite, bronz- 
itite, hypersthenite, hornblendite. 

Occurrence. The pyroxenites are usually found in 
association with gabbro and norite masses. Peridotites 
may have a similar occurrence. The serpentine deposits 
of Quebec and New England occur in this association. 
Serpentine asbestos is extracted in commercial quantities. 



158 OPTICAL MINERALOGY AND PETROGRAPHY 



CHAPTER 10. 

IGNEOUS ROCK TYPES. 

Volcanic Rocks. 
THE RHYOLITE FAMILY. 

Mineralogical Composition. Orthoclase, oligoclase, 
quartz. Biotite, hornblende. The rhyolites are chem- 
ically the equivalents of the granites, particularly the 
alkali-lime type. 

Texture. Well developed groundmass, often largely 
glassy. Frequently porphyritic. 

Relationship. Rhyolite grades imperceptibly into 
trachyte, granite, and dacite. Unless quartz is recog- 
nized, a microscopic examination is necessary to dis- 
tinguish rhyolite. It may easily be confused with dacite 
unless the polysynthetic twinning of the feldspar char- 
acteristic of dacite can be seen. 

Character. The term "rhyolite" comes from the 
Greek verb rhein, "to flow," because of the flow structure 
frequently observed. Liparite is a synonymous term 
used largely in Europe. It was named from the Lipari 
Islands, in Sicily. Quartz porphyry is a term often 
applied to the rhyolites which have crystallized as 
intruded sheets, laccoliths, dikes, and sills. The glassy 
portion is characterized by its behavior between crossed 
nicols, remaining dark during a complete revolution of 
the stage. 



IGNEOUS ROCK TYPES VOLCANIC ROCKS 159 

The processes of weathering of the rhyolites are the 
same as take place in granites, ordinary decomposition by 
atmospheric agencies giving rise to the formation of the 
hydrous aluminum silicates. Metamorphic processes 
develop schistose textures leading in extreme cases to 
the development of sericite schists. 

Early in the study of volcanic rocks it was customary 
to distinguish two types those which had erupted pre- 
vious to Tertiary times, and those which had erupted after 
Tertiary times. The former were called Paleovolcanic, 
and the latter were called Neovolcanic. Fortunately, this 
classification did not survive. 

Classification. Rhyolites are regarded by some writ- 
ers as porphyritic rocks with phenocrysts of quartz and 
alkali feldspars in a groundmass which is wholly glassy 
or a very finely crystalline aggregate of quartz and feld- 
spar. They classify in the "glasses" all varieties of vol- 
canic rocks in which chilling has prevented crys- 
tallization. 

The classification here adopted combines the glasses 
with the rhyolites. 

Volcanic glasses are obsidian, pumice, pitchstone, and 
perlite. 

OBSIDIAN is a dense, homogeneous glass with a low 
percentage of water. 

PUMICE is a cellular glass formed by the expansion 
of the cooling magma by the escaping steam bubbles. It 
is light, very porous, and may resemble blast-furnace 
slag. 

PITCHSTONE is essentially the same as obsidian, with 
a higher percentage of water. It is more resinous in 
appearance, giving it a greasy or pitchy luster. 



160 



OPTICAL MINERALOGY AND PETROGRAPHY 



PERLITE is a pitchstone which has a spheroidal 
arrangement of the particles, giving rise to a rounded 
fracture. 

Pantellerite. Pantellerite is a volcanic rock corre- 
sponding to the alkali granites. It is rare, and occurs 
so far as known only on the island of Pantellerea, in the 
Mediterranean Sea. It contains a rare feldspar called 
anorthoclase, which is an isomorphous mixture of albite 
and orthoclase. 

Distribution. Rhyolites are widespread throughout 
the Western States. Obsidian Cliff in Yellowstone Park, 
Silver Cliff in Utah, extinct volcanoes in New Mexico, 
Utah, Montana and California (Mono Lake), are well- 
known examples. In Leadville, Colorado, they are asso- 
ciated with the ore deposits. 

Along the Eastern Coast, remnants of rhyolite lavas 
from ancient Pre-Cambrian volcanoes have been found 
in New Brunswick, Maine, Massachusetts, and Pennsyl- 
vania. 



Analyses : 



Si0 2 



Fe.0, . 
FeO 
MgO . 
CaO 
Na z O . 

H,0 . 

Total 
Sp. Gr. 



123 

83.59 77.00 75.60 

5.42 12.80 11.50 

1.90 2.40 



30 

3.44 1.40 .80 

5.33 3.00 2.90 

1.37 4.10 5.90 

.76 .70 1.00 



4 

68.30 

10.90 

3.70 

.40 

.20 

1.40 

7.10 

4.10 



99.91 101.20 100.10 101.10 
2.54 2.41 2.44 2.48 



IGNEOUS ROCK TYPES VOLCANIC ROCKS 161 

1. Soda rhyolite from Berkeley, California. 

2. Liparite from Telkebanya, Hungary. 

3. Rhyolite from Hot Springs Hills, Pahute Range, Utah. 

4. Pantellerite, Kahania, Island of Pantelleria, Mediterranean. 

Discussion of Analyses: 

1. Rhyolites have the highest silica content of any 
volcanic rock and generally higher than granite. 

2. They have low iron, magnesia and lime contents, 
due to the scarcity of dark minerals. The lime comes 
from the acid plagioclase. 

3. Potash generally predominates over soda. 

4. In pantellerite, soda predominates over potash, due 
to the presence of anorthoclase. 

THE TRACHYTE FAMILY. 

Mineralogical Composition. Glassy orthoclase (sana- 
dine). Biotite, hornblende, augite, diopside. Magnetite 
and titanite as common accessories. Volcanic equivalent 
of the syenites. 

Texture. Groundmass usually crystalline of sana- 
dine, containing sanadine or orthoclase phenocrysts in 
which Carlsbad twinning is evident. Flow structure 
often conspicuous. 

Relationship. Trachytes pass into phonolites with 
increase in soda. They grade into syenites with the devel- 
opment of granitoid texture. They may be confused with 
andesites unless the striated feldspar of the latter is dis- 
tinguishable. 

Character. The name trachyte is derived from a 
Greek work trachus, meaning "rough," because of the 
rough character of the first rocks of this type which were 
studied. They are not common, and are found in the fol- 
lowing type localities: in the volcanic districts of Italy 



162 OPTICAL MINERALOGY AND PETROGRAPHY 

and the Auvergne, along the Rhine, in the Azores, in the 
Black Hills, in Ouster County, Colorado, and in Montana. 

Analyses of Trachytes (See under Phonolites). 
Discussion of Analyses. Compared with rhyolites, 
the trachytes show : 

1. Lower silica, due to decrease in quartz. 

2. Higher alumina, magnesia, lime and iron, due to 
increase in ferro-magnesian minerals. 

3. Higher alkalies, potash usually predominating. 

THE PHONOLITE FAMILY. 

Mineralogical Composition. Sanadine and nephelite 
or leucite. Aegirite. Occasionally members of the soda- 
lite group. Garnet as an accessory. 

Texture. The groundmass is crystalline, sometimes 
porphyritic, rarely glassy. 

Relationship. Phonolite grades into trachyte with 
decrease in soda. The two types are closely associated. 

Character. Phonolite is a translation into Greek of 
a German word Klingstein, or "cluck stone," so named 
because certain phonolites with a pronounced horizontal 
jointing when hit give forth a metallic sound. The rock 
has a greasy appearance, due to the presence of nephe- 
lite. Nephelite if identified serves at once to distinguish 
phonolite from other volcanic rocks. 

Leucite phonolites are rare. Leucite may and fre- 
quently does occur with nephelite in the typical phonolite. 
Concentrically arranged inclusions of magnetite specks 
occur in the leucite. 

The pyroxenes are more common than in any other 
volcanic rock. They are usually segirite-augite or segirite 
in long-tufted, ragged, bright green prisms. The acces- 



IGNEOUS ROCK TYPES VOLCANIC ROCKS 



163 



sory minerals sodalite, brown garnet, and titanite are in 
themselves characteristic. 

Phonolites are relatively not common. They occur in 
dikes, sheets and isolated buttes (Devil's Tower) in the 
Black Hills and in the Cripple Creek mining districts of 
Colorado, where they are associated with purple fluorite 
and calaverite in the ore bodies. The phonolite magmas 
being rich in alkalies may have had a solvent effect upon 
the gold, thus accounting for the present association. 

In Germany, phonolites occur in great masses as vol- 
canic necks or plugs in southern Baden, near the Swiss 
border. Many old castles have been erected on the 
summits. 

Kilimanjaro, one of the volcanoes which has recently 
been active, is said to have given forth phonolite lavas. 



Analyses of Trachytes and Phonolites: 



TRACHYTES. 



PHONOLITES. 



SiO, 
Al-O, 



FeO 
MgO 
CaO 
Na,0 

H = O 

Total 
Sp. Gr. 



1 


2 


3 


1 


2 


3 


66.30 


64.70 


66.03 


58.20 


58.50 


61.08 


17.80 


16.50 


18.49 


21.60 


19.70 


18.71 


2.30 


.70 


2.18 


21.80 


31.40 


1.91 


.40 


2.70 


.22 


.... 


.... 


.63 


.30 


1.70 


.39 


1.30 


.30 


.08 


2.10 


3.20 


.96 


2.00 


1.50 


1.58 


5.60 


2.70 


5.23 


6.00 


10.00 


8.68 


3.50 


5.50 


5.86 


6.60 


4.70 


4.63 


.20 


1.60 


.85 


2.10 


1.00 


2.21 



. 98.50 99.30 100.20 
2.6 2.56 . 2.59 



97.56 99.10 99.51 
2.6 2.58 



TRACHYTES. 

1. Trachyte from Auvergne, France. 

2. Biotite hypersthene trachyte from Tuscany, Italy. 

3. Trachyte from Game Ridge, Custer County, Colorado. 



164 OPTICAL MINERALOGY AND PETROGRAPHY 

PHONOLITES. 

1. Phonolite from Schlpssberg, Teplitz, Bohemia. 

2. Phonolite from Miaune, France. 

3. Phonolite from "Devil's Tower," Black Hills, Wyoming. 

Discussion of Analyses. Compared with trachytes, 
the phonolites show : 

1. Lower silica due to the substitution of nephelite 
for sanadine. 

2. Higher alumina and alkalies. 

3. Lower iron, magnesia and lime, due to absence of 
dark minerals. In case aegirite is present, soda and iron 
are increased. 

4. Traces of chlorine and sulphur are due to the pres- 
ence of members of the sodalite group. 

THE DACITE AND ANDESITE FAMILY. 
Mineralogical Composition. Acid plagioclase ; biotite, 
hornblende, augite, diopside ; magnetite, apatite, zircon as 
common accessories. Quartz is present in dacite and 
absent in andesite. 

Texture. Groundmass present as glass or as an inti- 
mate mixture of minute indistinguishable feldspars, 
which may be described as a "pepper and salt" texture. 
Plagioclase feldspar shows irregular outlines with zonal 
arrangement of inclusions frequent. 

Character. This group is the volcanic equivalent of 
the quartz diorite and diorite group. The dacites are not 
common. They were named from an old Roman province 
of Dacia, now a part of Hungary. Andesites derived 
their name from the abundance of lava of this type in 
the Andes Mountains. 

Differentiation from other volcanic rocks may be 
based upon the peculiar "pepper and salt" texture. Da- 



IGNEOUS ROCK TYPES VOLCANIC ROCKS 165 

cite and rhyolite are confused unless the twinning of the 
plagioclase of the former is observed. 

Among some of the active volcanoes which furnish 
andesite lavas are : Chimborazo, in the Andes ; Aphro- 
essa in the Santorin Archipelago, Aegean Sea, which was 
in eruption in 1863; Krakatoa, whose last eruption was 
in 1883, and the extinct volcanoes Mount Shasta, Mount 
Hood and Mount Rainier. 

Classification of the Andesites: 

1. Mica andesite. 

2. Hornblende andesite. 

3. Pyroxene andesite. 

a. Augite andesite. 

b. Hypersthene andesite. 

Analyses of Dacite and Andesite (See next table). 

Discussion of Analyses. Compared with trachytes 
the analyses show : 

1. Lower silica, due to the substitution of acid plagio- 
clase for alkali feldspar. 

2. Higher alumina, iron, magnesia and lime, due to 
the presence of the dark minerals. 

3. Lower alkalies. Soda always predominates over 
potash. This is due to the presence of the acid 
plagioclase. 

THE BASALT FAMILY, INCLUDING DIABASE. 
Mineralogical Composition. Basic plagioclase, au- 
gite; magnetite is a common accessory. Olivine is pres- 
ent in olivine basalt. 

Texture. These rocks possess a texture that is char- 
acteristic of rapid cooling. They have occasionally a 
glassy groundmass dotted with skeleton crystals, but more 
commonly they are porphyritic. The more crystalline 



166 OPTICAL MINERALOGY AND PETROGRAPHY 

portion of the rock consists of prominent augite and oliv- 
ine crystals with good outline, and with plagioclase poorly 
developed in small crystals. The pale buff color of the 
augite phenocrysts is characteristic. 

The texture of diabase is intermediate between that 
of gabbro and of basalt. It is essentially an intrusive 
basalt, entirely crystalline and granitoid. The plagio- 
clase crystals are idiomorphic, occurring in long, lath- 
shaped crystals which lie in all positions. The interstices 
are filled with allotriomorphic crystals of augite and mag- 
netite. This texture is called "ophitic." It is an impor- 
tant microscopic criterion for the identification of 
diabase. 

Diabase is variously classified, sometimes as a plutonic 
rock and sometimes as a volcanic rock. Since it grades 
into porphyritic forms at the contacts and since it is 
really volcanic in its nature, occurring in sheets or 
dikes of limited thickness close to the surface, it is 
considered here with the basalts. 

Basalts are volcanic equivalents of the gabbros. They 
are difficult to classify in the field, as they are all heavy, 
black gray or brown rocks for which a common and use- 
ful field term "dolerite" has been applied. The term dia- 
base originated from the Greek verb diabaitiein, "to pen- 
etrate." Trap is a common field synonym applied to 
rocks of diabasic texture. 

Classification. A simple classification of the basalt 
family which meets all field requirements is : 

1. Basalt. 

2. Olivine-basalt. 

3. Diabase. 

4. Olivine-diabase. 

Distribution. Basalts are abundant particularly along 
the Atlantic seacoast where diabase has intruded Triassic 



IGNEOUS ROCK TYPES VOLCANIC ROCKS 



167 



shales. It has formed prominent landmarks, such as the 
Palisades of the Hudson, East and West Rock near New 
Haven, and Deep River, North Carolina. Thousands of 
feet of basalt of Pre-Cambrian age are found on Kewee- 
naw Point. Native copper, secondarily precipitated in 
the amygdaloidal cavities of these flows, is an important 
ore. The Columbia Plateau and the Deccan Plateau fur- 
nish the two greatest examples of basaltic extrusion. 

The lavas from many volcanoes are chiefly basaltic. 
Among these are Kilauea and Mauna Loa in the Hawaiian 
Isands, Mount Etna, and various volcanoes in Iceland. 



Analyses of Dacite, Andesite, Basalt and Diabase: 



SiO, 
A1.CX 



FeO 

MgO 

CaO 

Na 2 

K 2 

H 2 

Total 



1 


2 


3 


4 


5 


6 


69.40 


62.00 


60.30 


51.80 


49.70 


49.20 


16.20 


17.80 


16.90 


12.80 


13.60 


13.50 


.90 





5.90 


3.60 


7.80 


5.50 


1.50 


4.40 


1.40 


8.70 


7.20 


10.60 


1.30 


2.60 


3.50 


7.60 


5.50 


6.80 


3.20 


5.40 


5.60 


10.70 


12.40 


11.50 


4.10 


4.30 


3.80 


2.10 


1.60 


1.80 


3.00 


1.50 


2.40 


.40 


1.20 


.10 



.40 



1.70 



.40 



.60 



.10 



.30 



100.00 100.99 100.20 98.30 



99.10 99.80 



1. Dacite from Lassen's Peak, California. 

2. Hypersthene andesite from Mount Shasta, California. 

3. Augite andesite from Chimborazo, Mexico. 

4. Diabase from New Haven, Connecticut. 

5. Basalt lava from Thjorsa, Iceland. 

6. Iron basalt from Nifak, Disco Island, Greenland. 

Discussion of Basalt Analyses. When compared with 
andesites, the analyses show: 

1. Lower silica. 

2. Higher alumina. 



168 OPTICAL MINERALOGY AND PETROGRAPHY 

3. Lower alkalies, all due to the increasing basicity 
of the feldspar. 

4. Higher iron, magnesia and lime, due to the addi- 
tion of dark minerals. 

5. Soda predominates over potash. 

TRACHYDOLERITES. 

Mineralogical Composition. Basic plagioclase and 
alkali feldspar ; pyroxene ; members of the sodalite group, 
olivine, and hornblende. 

Texture. Often porphyritic, with phenocrysts of 
basic plagioclase. 

Relationship. Trachydolerites are the volcanic equiv- 
alents of essexites. They are intermediate between alkali 
trachytes and phonolites on one side and tephrites on the 
other. 

Analyses (See next table). The analyses show low 
silica, high alumina, iron, magnesia, lime, and the 
alkalies. 

TEPHRITES AND BASANITES. 

Mineralogical Composition. Basic plagioclase and 
either nephelite or leucite. Augite is common. Basanite 
contains olivine and tephrite does not. 

Relationship. These rocks are the volcanic equiva- 
lents of the theralites and the shonkinites. 

Classification. Basic plagioclase is common to all 
varieties. 

1. Leucite tephrite, with addition of leucite and au- 
gite. 

2. Leucite basanite, with addition of leucite, augite 
and olivine. 

3. Nephelite tephrite, with addition of nephelite and 
augite. 



IGNEOUS ROCK TYPES VOLCANIC ROCKS 169 

4. Nephelite basanite, with addition of nephelite, 
augite and olivine. 

The lavas of Mount Vesuvius are leucite tephrites, 
containing a little olivine, thus grading toward the leu- 
cite basanites. 

Analyses (See next table). The chemical character of 
this group is very similar to that of the last group. The 
alkalies are higher, due to the addition of leucite and 
nephelite. 

LEUCITITES AND LEUCITE BASALTS. 
NEPHELITITES AND NEPHELITE BASALTS. 

Mineralogical Composition. Leucite or nephelite 
with augite; with or without olivine. Nonfeldspathic. 
They grade from tephrites and basanites by the elimina- 
tion of the basic feldspar. 

Relationship. These rocks are the volcanic equiva- 
lents of the ijolites and the missourites. The nephelite 
basalts accompany the phonolites in the Cripple Creek 
district of Colorado, and are sometimes completely 
impregnated with gold telluride. 

Analyses (See next table). The chemical character 
of this group corresponds closely to that of the tephrite 
and basanite group if the characteristics normally con- 
tributed by the basic plagioclase are deducted. 

LIMBURGITE AND AUGITITE. 

Mineralogical Composition. Limburgite consists of 
pyroxene and olivine, and augitite consists of pyroxene 
only. They both contain apatite and magnetite as com- 
mon accessories. They contain no feldspars nor feld- 
spathoids. 

Relationship. These rocks are regarded as the most 
basic of the volcanic rocks and are the end products of 
the trachydolerite-tephrite-leucitite series. They may be 



170 OPTICAL MINERALOGY AND PETROGRAPHY 

regarded as the volcanic equivalents of peridotite and 
hornblendite. 

Analyses. Chemically they show a decrease in the 
alkalies, due to the absence of the feldspathoids. Lim- 
burgite shows an increase in magnesia, due to the pres- 
ence of olivine. 



SiO, 

A1 2 O; 

Fe 2 : 

FeO 

MgO 

CaO 

Na.O 

K 2 

H 2 



1 


2 


3 


4 


5 


6 


7 


51.76 


48.09 


47.40 


45.90 


46.90 


42.78 


43.35 


16.64 


20.12 


23.70 


18.70 


21.60 


8.66 


11.46 


14.06 


6.72 


6.80 


.... 


8.10 


.... 


11.98 




4.32 


3.50 


10.70 


.... 


17.96 


2.26 


3.21 


4.19 


1.90 


5.70 


2.50 


10.06 


11.69 


8.15 


9.37 


6.50 


10.60 


8.00 


12.29 


7.76 


4.98 


2.62 


6.40 


1.70 


8.90 


2.31 


2.88 


1.31 


5.69 


3.30 


6.80 


2.60 


.62 


.99 






1.70 


.60 


2.10 


3.96 


2.41 



Total . 100.11 101.12 101.20 100.60 100.10 98.64 95.80 

1. Trachydolerite from Chajorra, Island of Teneriffe. Erup- 

tion of 1798. 

2. Leucite tephrite from Atreo del Cavallo, Vesuvius. Erup- 

tion of May, 1855. 

3. Nephelite tephrite from lava stream on San Antao, Cape 

Verde Islands. 

4. Leucitite from Capo de Bova, Via Appia, Rome. 

5. Nephelinite from San Antao, Cape Verde Islands. 

6. Limburgite from Limburg, Kaiserstuhl Mountains, Baden, 

Germany. 

7. Augitite from Hutberg, near Tetscfcen, Germany. 

PYROCLASTIC ROCKS. 

Pyroclastic rocks are those made up of fragmental 
volcanic deposits, usually more or less stratified by trans- 
portation through the air or under water. 

Classification. 1. Volcanic agglomerate consists of 
the large sized volcanic products of a fragmental nature 
which have been deposited near the crater. 



IGNEOUS ROCK TYPES VOLCANIC ROCKS 171 

2. Volcanic breccia consists of angular fragmental 
products which have been firmly cemented together. 

3. Tuff is a deposit of volcanic ash or dust consoli- 
dated by cementation. In the historical eruption of 
Vesuvius, in 79 A.D., Pompeii was buried in ash and Her- 
culaneum was buried in tuff. Excavation has gone on 
rapidly in Pompeii, as the loose ash offers less resistance 
to removal than does the tuff which covered Herculaneum. 



172 OPTICAL MINERALOGY AND PETROGRAPHY 



CHAPTER 11. 
SEDIMENTARY AND METAMORPHIG ROCKS. 

Sedimentary rocks are of secondary origin in that 
they are formed from previously existing rocks which 
may have been either igneous, sedimentary, or metamor- 
phic. They may be mechanically or chemically deposited, 
either on land or under water. The agents of deposi- 
tion are water, wind, and ice. 

By the weight of overlying strata and through the 
agency of siliceous, calcareous or ferruginous cement, 
they become consolidated from a loose aggregate to a 
solid mass. 

CLASSIFICATION. 1. Sediments of mechanical origin. 

A. Water deposits. 

a. Conglomerate. 

b. Breccia. 

c. Sandstone. 

d. Shales. 

B. Wind deposits. 

a. Loess. 

b. Sand dunes. 

2. Sediments of chemical origin formed from 
solution. 

A. Concentration, 
a. Sulphates. 
Gypsum. 
Anhydrite. 



SEDIMENTARY AND METAMORPHIC ROCKS 173 

b. Chlorides. 
Halite. 

c. Silica. 
Flint, etc. 

d. Carbonates. 
Limestone, etc. 

e. Oxides. 

Iron Ores. 

B. Organic, through the agency of animals and 
plants. 

a. Carbonates. 

Limestones of several kinds. 

b. Silica. 
Diatomaceous earth, etc. 

c. Phosphate. 

Phosphate rock. 

d. Carbon. 

Coal, etc. 
Sedimentary Rocks of Mechanical Origin. 

Conglomerates. Conglomerate is a rock consisting of 
cemented fragments of rounded, water-worn material of 
varying sizes. The pebbles are usually made of the more 
resistant varieties of minerals and rocks. Finer sedi- 
ments fill the interstices. Conglomerates are aqueous in 
origin and show more or less stratification. The repre- 
sent near-shore conditions of sedimentation. 

Breccias. A breccia is a rock composed of angular 
fragments cemented into a solid mass. 

1. Talus breccia is derived by ordinary weathering 
and disintegration of a rock ridge. It accumulates at 
the base of slopes or cliffs. 

2. Fault or friction breccia is derived from earth's 
movements, which crush the rock on two sides of a fault 
plane by friction or by intense pressure. 



174 OPTICAL MINERALOGY AND PETROGRAPHY 

3. Volcanic breccia is formed by an accumulation of 
angular fragments ejected by volcanic action and later 
solidified. 

Sandstone. Sandstone is composed of sand grains 
which have been rounded by water action and separated 
by the classifying action of moving water to deposits of 
uniform texture. Quartz is the essential constituent, 
although impurities are always present, such as feldspar, 
mica, garnet, magnetite, etc. 

CLASSIFICATION. According to the character of the 
cement : 

1. Siliceous sandstone. 

2. Ferruginous sandstone. 

3. Calcareous sandstone. 

4. Argillaceous sandstone. 

According to mineral content : 

1. Arkose, containing much feldspar.- 

2. Graywacke, containing ferro-magnesian min- 
erals. 

3. Micaceous sandstone, etc. 

Shale. Shale is a rock consisting of the finer mate- 
rial, usually clayey, deposited beyond the zone of depo- 
sition of the sandstone. It contains compacted clays, 
muds and silts, which possess a finely stratified structure 
called bedding. 

CLASSIFICATION. According to composition. 

1. Argillaceous shales. 

2. Arenaceous shales. 

3. Ferruginous shales. 

4. Carbonaceous shales. 

Shales form about 87 per cent of the sedimentary 
rocks, sandstones about 8 per cent, and limestones about 
5 per cent. 



SEDIMENTARY AND METAMORPHIC ROCKS 175 

Loess. Loess is a fine, homogeneous, clay-like sub- 
stance, largely siliceous, which lacks all semblance of 
stratification, and when eroded forms precipitous cliffs. 
It contains angular quartz, mica flakes, clayey material, 
with often high percentages of calcium carbonate. 

Loess is believed to be a wind-blown deposit, probably 
assisted in certain localities by aqueous deposition. It is 
used in the West for brick manufacture. 

Adobe clay is a form of loess abundant in the arid 
southwestern portion of the United States. It is used in 
the manufacture of sun-dried brick for adobe houses. 

Sand Dunes. Sand dunes are formed by wind-blown 
sand, which always exhibits a characteristic shape with 
its long, gentle slope on the windward side, up which the 
sand grains are blown, and with a steeper slope on the 
leeward side, which is the angle of repose for sand grains. 
Sand dunes show stratification and ripple marks. 

The migration of sand dunes has been known to create 
havoc in certain parts of the coutry. They are "fixed" 
by transplanting with beach grass and sand hedges. One 
of the railroads temporarily checked the progress of some 
advancing sand dunes by spraying them with crude 
petroleum. 

Sediments of Chemical Origin. 

Gypsum and Anhydrite. Gypsum and anhydrite, the 
hydrous and the anhydrous sulphates of lime, occur inter- 
bedded or in irregular masses, interstratified with clays, 
shales, sandstones, and limestones, or with halite. 

They originate from concentration of oceanic waters 
by evaporation, and in inland lakes in which the evapora- 
tion equals or exceeds the amount of inflow. 

Anhydrite changes to gypsum by normal hydration, 
due to exposure. A tunnel in Europe which was driven 



176 OPTICAL MINERALOGY AND PETROGRAPHY 

through anhydrite was thrown out of alignment by the 
volume increase produced by this change. 

Halite. Halite occurs in massive, granular form, 
interstratified with clays, marl and sandstone. It is espe- 
cially associated with gypsum, anhydrite and dolomite. 
The deposits of Strassford, Germany, are 4,000 feet thick. 
It is here associated with the chlorides and sulphates of 
potassium and magnesium. 

Flint. Flint is a cryptocrystalline variety of silica 
occurring as a hard, grayish to blackish rock, its color 
being due to carbonaceous matter. It occurs as nodules 
or lenses in limestones. It is used for road material, and 
in tube and ball mills. 

Sediments of Organic Origin. 

Limestone. Limestone is a widely distributed cal- 
cium carbonate rock containing impurities of magnesia, 
silica, clay, iron, and organic matter. It is quite soluble, 
and allows the formation of sink holes, caves, solution 
cavities, etc. Buildings erected of limestone thirty or 
forty years ago often show the effect of weathering by 
pitted and etched surfaces. 

Limestone forms by chemical precipitation and 
through the agency of animals and plants. 

CLASSIFICATION. The classification of limestone is 
based upon composition, texture, and uses. It has a wide 
range of occurrence. 

1. Calcic limestone is chiefly calcium carbonate. 

2. Dolomite refers usually to any magnesian rich 
limestone. 

3. Chalk is a soft, porous, fine-grained variety com- 
posed of minute shells of foraminifera. 

4. Hydraulic limestone is a clayey limestone used in 
cement manufacture. 



SEDIMENTARY AND METAMORPHIC ROCKS 177 

5. Lithographic limestone is a fine-grained, homo- 
geneous variety used for lithographic work. 

6. Oolitic limestone is composed of small spherical 
grains of calcium carbonate. 

7. Travertine is a porous, cellular variety deposited 
by hot springs. 

Iron Ores. The oxides of iron, hematite and limonite, 
and the iron carbonate, siderite, may all have a sedimen- 
tary origin, either secondary or primary. They are all 
commercially valuable as a source of iron. 

Phosphate Rock. Phosphate rock consists chiefly of 
calcium phosphate. It has a value as a source of phos- 
phoric acid in the manufacture of fertilizers. It is of 
organic origin, formed from animal remains. Large 
deposits are found in Florida, Tennessee, Idaho, Wyo- 
ming, and Utah. 

Carbonaceous Rocks. These rocks include all accu- 
mulations of vegetable matter that have undergone par- 
tial or complete decay under water. 

The principal varieties which form transitional stages 
from the unaltered plant remains to graphite by steadily 
increasing metamorphism are peat, lignite or brown coal, 
bituminous or soft coal, and anthracite or hard coal. 

Metamorphic Rocks. 

Agents of Metamorphism. The chief agents involved 
in the alteration of igneous and sedimentary rocks to 
their metamorphic equivalents are: (1) dynamic action 
due to earth movements producing shearing and folding 
of the rock formations, and (2) chemical action influ- 
enced by heat, liquids, and gases. 

Composition of Metamorphic Rocks. The chemical 
composition of metamorphic rocks is frequently similar 



178 OPTICAL MINERALOGY AND PETROGRAPHY 

to the composition of the rocks from which they are 
derived. In so far as this is true, chemical analysis is 
an important criterion for discriminating between meta- 
morphosed sedimentary and metamorphosed igneous 
rocks. 

Frequently there is addition or subtraction of con- 
stituents accompanying metamorphism which renders 
more difficult the interpretation of the origin of the meta- 
morphic rock. 

Mineral composition may or may not be the same in 
the metamorphic rock as it was in the original rock. Fre- 
quently metamorphism is accomplished by a granulation 
and rotation of the original particles. In the greater 
number of cases, there is a development of platy minerals 
which are best adapted to withstand conditions of higher 
pressures and temperatures. In these minerals the mutual 
parallelism of the greatest, mean and least dimensional 
axes causes a more or less perfect cleavage in one plane, 
which is called schistocity. The average ratio of the 
greatest to the mean dimensions of mica is 10 : 1, of 
hornblende 4 : 1, and of quartz and feldspar 1.5 : 1. 

A metamorphic rock contains a higher percentage of 
.the minerals mica and hornblende than the original rock. 
For example, shale may contain no mica. By metamor- 
phism, mica schist is developed, containing over 50 per 
cent of mica. No change in chemical composition has 
taken place. Obviously the mica was developed by a 
recrystallization of the constituents originally contained 
in the rockmass. 

Minerals which are characteristic of metamorph : c 
rocks are staurolite, cyanite, sillimanite, zoisite, chlorite, 
talc, etc. Quartz, feldspar, mica, pyroxene, and amphi- 
bole are common to both igneous and metamorphic rocks. 



SEDIMENTARY AND METAMORPHIC ROCKS 179 

Criteria for the Discrimination of Metamorphosed 
Igneous and Metamorphosed Sedimentary Rocks: 

1. Mineralogical composition. 

The minerals which are strongly indicative of a sedi- 
mentary origin of the metamorphic rocks in which they 
occur are staurolite, andalusite, sillimanite, cyanite. They 
all contain higher percentages of alumina than those 
found in igneous rocks, and as alumina is almost insoluble 
there is practically no possibility of an addition of alu- 
mina from other sources. 

2. Original textures and structures. 

If not too severely metamorphosed, sedimentary rocks 
may show remnants of bedding, fossils, cross-bedding, or 
other features. Igneous rocks may show amygdaloidal 
cavities, flow structure, etc. 

3. Field relationships. 

Areal distribution and association of the metamor- 
phosed rock with surrounding rocks may give some clue. 
By tracing the metamorphosed rock laterally along the 
strike, one may come to a less metamorphosed portion of 
the rock which still shows original sedimentary textures 
or structures. 

4. Chemical composition. 

a. Dominance of magnesia over lime is indicative of 
sedimentary origin. 

b. Dominance of potash over soda is suggestive of 
sedimentary origin. 

c. The presence of several per cent of alumina over 
the 1 : 1 ratio necessary to satisfy the lime and alkalies 
is suggestive of sedimentary origin. 

d. A high silica content is suggestive of sedimentary 
origin if supported by other criteria. 



180 OPTICAL MINERALOGY AND PETROGRAPHY 

CLASSIFICATION. The classification of metamorphic 
rocks is based upon composition, texture, and structure : 

1. Gneisses. 

2. Schists. 

3. Quartzites. 

4. Slates and phyllites. 

5. Marbles. 

6. Ophicalcite, serpentine, and soapstone. 

TABLE OF SEDIMENTARY ROCKS AND THEIR 
METAMORPHIC EQUIVALENTS. 



Loose Sediments 
Gravel. 
Sand. 
Silt and clay. 
Lime deposits. 


Consolidated 
Rock 
Conglomerate. 
Sandstone. 
Shale. 
Limestone. 


Metamorphic 
Rock 
Gneiss, schist. 
Quartzite. 
Slate, phyllite. 
Marble. 



TABLE OF IGNEOUS ROCKS AND THEIR 
METAMORPHIC EQUIVALENTS. 

Igneous Rocks. 

Coarse-grained feldspathic Metamorphic Rocks. 
rocks, as granite, syenite, 
etc. Gneiss. 

F i n e-grained feldspathic 

rocks, as tuff, etc. Gneiss ' schlst 

Basic igneous rocks, as , . 

diorite, basalt. 

Gneiss.- -A gneiss is a banded metamorphic rock, 
either of igneous or sedimentary origin, in which the 
bands are mineralogically unlike, consisting chiefly of 
quartz and feldspar, with or without the parallel dimen- 
sional arrangement necessary for rock cleavage. 

Gneisses are developed by a granulation, rotation and 
recrystallization of the original minerals rather than by 



SEDIMENTARY AND METAMORPHIC ROCKS 181 

the development of an entirely new set of the platy, cleav- 
able minerals. 

CLASSIFICATION. According to the prevailing acces- 
sory mineral: 

Biotite gneiss. 

Muscovite gneiss. 

Hornblende gneiss, etc. 
According to origin : 

Granite gneiss. 

Gabbro gneiss. 

Diorite gneiss, etc. 

Schist. A schist is a foliated, metamorphic rock 
whose individual folia are mineralogically alike, and 
whose principal minerals are the flat, platy minerals 
which are best adapted to withstand conditions of high 
pressure and high temperature. The parallel arrange- 
ment of the minerals develops the capacity to part along 
parallel planes, called schistosity. 

CLASSIFICATION. According to the prevailing schis- 
tose mineral : 

Chlorite schist. 

Mica schist. 

Talc schist. 

Actinolite schist, etc. 

Quartzite. Quartzite is developed from sandstone by 
a recrystallization of the original constituents into a hard, 
compact, crystalline mass having a splintery or con- 
choidal fracture. 

A pure quartzite is rare, although the percentages of 
alumina, iron and the bases are often small. With an 
increase in impurities, the quartzite tends to take on a 
schistocity, due to the formation of the impure constitu- 
ents by recrystallization into the flat, platy minerals. 
Quartzite schist is such a transition in which mica scales 



182 OPTICAL MINERALOGY AND PETROGRAPHY 

are found along the foliation planes. These planes prob- 
ably represent original bedding planes in the sandstone. 

Quartzite is used to advantage as a building stone, 
although its extreme hardness is found to be a handicap 
both in quarrying and in dressing. 

Slate and Phyllite. Slate is a dense, thinly cleavable, 
homogeneous rock, whose cleavage pieces are mineral- 
ogically unlike, and whose mineral grains are so small in 
size as not to be distinguished by the eye. This cleavage 
is not to be confused with original bedding planes. It is 
a secondary structure produced in the development of 
the secondary minerals. 

Slates are composed of the finest particles of mineral 
matter which are carried in suspension and deposited 
considerable distances from shore. Volcanic ash and tuff 
more rarely give rise to slate deposits. 

Phyllite is the next step in the metamorphism of a 
slate, intermediate between slate and mica or sericite 
schist. Quartz and mica are the essential minerals. 

Marble. Marble is the metamorphic equivalent of a 
limestone or a dolomite. It is completely recrystallized, 
and when pure shows the development of large rhombic 
calcite crystals or fine sparkling surfaces. 

Few original limestones are pure. The metamor- 
phism of an impure limestone containing silica, clayey 
material, iron oxides and carbonaceous matter is charac- 
terized not only by the recrystallization of calcium car- 
bonate but by the development of various secondary sili- 
cates, particularly biotite, wollastonite, diopside, tremo- 
lite, actinolite, grossularite, and hornblende. At least 
seventy secondary minerals have been found to exist in 
metamorphosed limestones. 

When pure, marble is massive, and shows no indica- 



SEDIMENTARY AND METAMORPHIC ROCKS 183 

tion of a schistose structure. All traces of fossils and 
original structures are obliterated. 

Ophicalcite. Ophicalcite is a variety of marble asso- 
ciated with streaks and spots of serpentine. Verde 
antique is a name more popularly used. It results from 
the metamorphism of an originally impure limestone to a 
calcite-silicate rock in which the silicates were later 
altered by hydration to serpentine. Ophicalcite is valu- 
able for decorative purposes, as it takes an easy polish. 
It occurs in quantities in Quebec, in the Green Mountains, 
and in the Adirondacks. 

Serpentine. Serpentine rock consists essentially of 
the mineral serpentine, a hydrous magnesium silicate, in 
association with olivine, pyroxene, hornblende, magnetite, 
chromite and the carbonates. Garnets and micas are 
common accessories. 

Serpentine is derived by metamorphism of igneous 
or other metamorphic rocks which are essentially com- 
posed of magnesium silicates, as olivine, pyroxene, or 
hornblende. Such rocks are basic igneous rock and horn- 
blende schist. 

Serpentine occurs in the crystalline area of eastern 
United States, in eastern Canada, and in a few of the 
western coast States, but seldom in large masses. It is 
used as an ornamental stone and as a source of asbestos. 

Soapstone. Soapstone is essentially the mineral talc. 
It becomes a talc schist by taking on a foliated structure. 
Impurities are mica, chlorite, tremolite, enstatite, mag- 
netite, quartz, and pyrite. 

Soapstone has a similar origin to serpentine as a sec- 
ondary product from the magnesium silicates. It is found 
in association with talcose and chloritic rocks in crystal- 
line areas. 



184 OPTICAL MINERALOGY AND PETROGRAPHY 

Soapstone is mined extensively in Virginia. The rock 
has many uses. It goes into the manufacture of tubs, 
switchboards, insulators, sinks, stoves, fire-brick and 
lubricants. 



SUGGESTIONS FOR GEOLOGICAL WORK 185 



APPENDIX. 

Suggestions for Geological Work. 

The necessity for constant and careful observation 
cannot be too insistently urged in an examination of rocks 
or geological structures, whether it be a prelimiary recon- 
noissance or a final detailed survey of a property of lim- 
ited extent. The geologist or mining engineer who is 
doing geological mapping should adopt the attitude that 
it may be impossible to return to the particular outcrop 
upon which he is working. Every feature of the rock 
which may be of possible value in the solution of the 
problem involved should be recorded on the spot. This 
outcrop may prove to be the keystone for the interpreta- 
tion of the structure of the entire area. A close applica- 
tion of this rule will save much useful time and energy. 

Observations for Geological Mapping. 
The following outline (from Farrell) may be used as 
a guide to the geological features to be observed in an 
examination of a 'property : 
A. RECONNOISSANCE OF THE AREA. 

1. Is the geology simple or complicated? 

a. Do the different formations cover a large or 

a small area? 

b. Is it easy to distinguish between them? 

c. Are the boundaries easy to find and follow? 

2. What are the probable rock types and their rela- 
tions ? 

a. Are rocks of igneous or sedimentary origin or 
both? 



186 OPTICAL MINERALOGY AND PETROGRAPHY 

b. Are contacts conformable or unconf ormable ? 

c. In case of intrusive bodies, are there large dikes 

or small masses and few in number, or are they 
small and widely distributed through the 
intruded formations? 

d. Are the rocks much altered? 

e. Is metamorphism a prominent feature? 

3. Collect specimens of the different formations, giv- 
ing locations as closely as possible. 

4. Note roads, trails, water, and possible camping 
places. 

B. GEOLOGICAL MAPPING GENERAL. 

1. Locate boundaries between formations. 

a. Simple boundaries. 
Take dip and strike. 

Does boundary indicate conformable or uncon- 

f ormable contact? 
Are there evidences of faulting? 

b. Obscure boundaries. 

Look for fragmental traces of the formations. 
Work up hill and locate the highest points at 

which fragments of the lower formation 

appear. 
Note whether scarps or change of slope are 

connected with the boundary. 

c. Complicated boundaries. 

Intrusive boundaries. 

Map carefully dikes and arms. 

Note alteration and metamorphism in the 

neighborhood of the boundary. 
Note variations in texture of the igneous 

rock in approaching the boundary. 



SUGGESTIONS FOR GEOLOGICAL WORK 187 

Boundaries showing contact metamorphism. 
Map the general relations of the metamor- 

phic patches. 
Note the metamorphic minerals and their 

succession. 

Note presence and association of ore min- 
erals. 

2. Work within the boundaries of a formation. Trav- 
ersing. 

a. In areas of sedimentary rocks. 

Strike and dip of beds. 

Color, thickness and general character of beds. 

Minerals composing the rocks; nature of the 
grains or fragments (angular or rounded) ; 
cementing material. In conglomerates look 
for recognizable fragments of earlier 
formations. 

Presence of fossils. 

Areas of alteration. 

Areas of metamorphism. 

Systems of folds minor folding direction and 
pitches of axes of folds relations of fold- 
ing to faulting. 

b. In areas of igneous rocks. 

Rock texture and variations in texture. 

Variations in composition. 

Segregations. 

Inclusions of other rocks. 

Dip and strike of schistosity or gneissoid 

structure. 
Flow structure. 

c. In areas of metamorphic rocks. 

Is rock of sedimentary origin? 
Does it show traces of bedding? 



188 OPTICAL MINERALOGY AND PETROGRAPHY 

Are gneissoid laminae continuous, sugges- 
tive of sheared beds ? 

Are the grains rounded or angular in out- 
line? What are their relative sizes? 

Are minerals such as to suggest erosion 

or metamorphic processes? 
Is the rock of igneous origin? 

Are the minerals typical of igneous rocks ? 

Are the gneissoid laminae noncontinuous, 

suggestive of sheared minerals? 
Are the changes suggestive of dynamic action, 

chemical action or both ? 

Is the rock texture suggestive of folding 
and shearing? 

Does it suggest an impregnation and meta- 
morphism by replacement process, due 
to action of solutions ? 

Is the rock widely different in structure and 
composition from the original type? 

C. GEOLOGICAL MAPPING DETAILED WORK. 

1. Faults. 

a. Strike and dip. 

b. Evidences of movement slickensides, striae 

(their direction and dip), gouge, drag, etc. 

c. Cross fracturing. 

2. Veins and other ore bodies. 

a. Strike and dip. 

b. General character of mineralization. 
Strong or weak. 

Oxidized or unoxidized vein material. 

c. Minerals and groups of minerals. 

d. Relative age of minerals. 



SUGGESTIONS FOR GEOLOGICAL WORK 189 

e. Represent exact outline of ore body as far as 

possible. 

f. Note occurrence of branches or false walls. 

g. Note character and extent of alteration of 
country rock. 

h. Nature and extent of replacement of the wall 
rock by the ore. 

Criteria of Relative Age. 

1. Older rocks are more likely to have been deformed 
and metamorphosed, and therefore are harder to 
recognize. 

2. Older formations are normally at the base of the 
series of sediments and flows. 

3. Older formations are represented by fragments in 
later formations. 

4. Younger formations fill erosion irregularities, 

fractures, fault planes and caves in older forma- 
tions. 

5. Younger formations cut the older ones as dikes, 
and include fragments of them. 

Table for the Examination of Rocks in the Laboratory. 

A. IGNEOUS ROCKS. 

1. Texture, historical deductions, etc. 

2. Mineralogical composition. 

a. Accessory minerals. 

b. Essential minerals. 

3. Relative age of minerals. 

a. Minerals formed with crystal boundaries are 
older than the surrounding ones without crys- 
tal boundaries. 



190 OPTICAL MINERALOGY AND PETROGRAPHY 

b. Included minerals are older than the ones which 
include them. 

c. Minerals abutting without crystal boundaries 

are of the same age approximately. 

d. Intergrown minerals are of the same age. 

e. Minerals cutting others are younger than those 

they cut. 
4. Alteration and metamorphism. 

a. Degree of change, extent to which original min- 

erals are changed. 

b. Character of the change, secondary minerals 
due to alteration, metamorphic minerals. 

B. SEDIMENTARY ROCKS. 

1. Relative sizes and shapes of the component par- 
ticles. 

a. Unassorted material, large and small fragments 
together, imply that the source of the material 
is near at hand or that the transporting agent 
is very powerful. 

b. Angular grains, fresh in appearance, indicate 

disintegration without decomposition, and 
little movement from the source. 

c. Rounded grains, fresh, imply disintegration 

and transportation of the material. 

d. Sorted material, where the grains are similar 
in mineralogical character, indicates that the 
deposits were made some distance from the 
source, or the original rock disintegrated and 
weathered also, differentiating the more resist- 
ing minerals. 



SUGGESTIONS FOR GEOLOGICAL WORK 191 

2. Look for fragments which give some clue as to the 

source from which the sedimentary material is 
derived. 

3. Determine the character and probable origin of 
secondary cementing material. 

C. WITH METAMORPHIC ROCK TRY TO DETERMINE : 

1. The nature of the original constituents and the 

original rock. Look for traces of original min- 
erals in form or cleavage. 

2. The nature of the alteration. 

a. Dynamic folding, shearing, etc., distortion of 

crystals or fragments. 

b. Chemical change older minerals partially dis- 

solved by later ones. 



INDEX. 



Absorption 66, 67 

Accessory minerals ...124, 129 

Acmite 99 

Actinolite 103 

Acute bisectrix 60 

Adjustment screws ....... 30 

Aegirite 99 

Albite 121 

Albite twinning 114 

Alkali granite 139 

Alkali-lime granite 139 

Alkali-lime syenite 145 

Alkali syenite 146 

Allotriomorphic 128 

Amphibole 100 

Analyses 

Andesite 167 

Augitite 170 

Basalt 167 

Dacite 167 

Diabase 167 

Diorite 150 

Essexite 154 

Gabbro 152 

Granite 144 

Ijolite 154 

Leucite syenite 148 

Leucitite 170 

Limburgite 170 

Malignite 154 

Missourite 154 

Nephelinite 170 

Nephelite syenite 148 

Norite . . 152 



Peridotite 156 

Phonolite 163 

Pyroxenite 156 

Rhyolite 160 

Shonkinite 154 

Syenite 146 

Tephrite 170 

Theralite 154 

Trachydolerite 170 

Trachyte 163 

Analyzer 26 

Andalusite 86 

Andesite 164 

Angle of incidence 15 

Anhydrite 175 

Anisotropic media. .14, 41, 75, 86 

Anorthite 122 

Anorthosite 151 

Apatite 82 

Appendix 185 

Arkose 174 

Assimilation 131 

Augite 98 

Augitite 169 

Axes of ether vibration . . . 

42, 49, 64, 66 

Basalt 165 

Basanite 168 

Bastite 91 

Baveno 114 

Becke method 38 

Bertrand lens 30, 51 

Biaxial crystals. 14, 48, 52, 64, 86 



194 



OPTICAL MINERALOGY AND PETROGRAPHY 



Biotite 107 

Birefringence . . 40, 44, 48, 50, 60 

Bisectrix 49 

Breccia 173 

Calcite 44, 80 

Canada balsam .36, 42 

Carlsbad 114 

Cementing material 174 

Centering screws 27 

Chalk 176 

Chlorite 108 

Cleavage 34 

Clinochlore 110 

Coal 177 

Compensation point 47 

Color 42, 44,45, 143 

Color scale 45 

Concentration 151 

Conglomerate 173 

Contact metamorphism 141 

Convergent lens 27 

Corundum 78 

Critical angle 16 

Cross hairs 27, 30 

Cryptocrystalline 128 

Crystal form 34 

Crystallization 128, 131 

Dacite 135, 164 

Diabase 135, 165 

Diallage 97 

Differentiation 131 

Diopside 96 

Diorite 149 

Dispersion 56 

Dolerite 166 

Dolomite 81, 176 

Double refraction. .18, 21, 40, 48 
Due de Chaulnes, method of 37 
Dunite .155 



Enstatite 89 

Epidote 110 

Essential minerals 124 

Essexite 152 

Extinction angle. 42, 64, 118, 119 
Extraordinary ray... 18, 22, 42 
Extrusive flow : 127 

Farrell 185 

Fault breccia 173 

Feldspar 43 

Felsitic texture 126 

Flint 176 

Fluorite 74 

Gabbro 150 

Garnet 72 

Garnierite 155 

Gases, influence of 132 

Geological mapping 185 

Geological observations . . . 185 

Glass 159 

Glassy texture 126, 128 

Gneiss 180 

Granite 138 

Granitoid texture 127 

Graphic granite 140 

Graywacke 174 

Greisen 140 

Gypsum 175 

Halite 176 

Hematite 77 

Hexagonal minerals 41, 77 

Holocrystalline 128 

Hornblende 103 

Hornblendite 157 

Hornfels 142 

Hypersthene 90 

Hypidiomorphic 128 

Hypocrystalline 128 



INDEX 



195 



Iceland spar 22 

Iddings 63 

Idiomorphic 128 

Igneous rocks .... 123, 137, 138 

Ijolite 154 

Ilmenite 78 

Immersion method 38 

Index of refraction 15, 36 

Interference 21, 42, 44,- 45, 48, 50 

Interference figures 51-56 

Intrusive 127 

Isometric minerals. 14, 40,69,70 

Isotropic media 13 

Kaolinite 112 

Kilauea 167 

Kilimanjaro 163 

Kimberlite 155 

La Crpix 56 

Labradorite 121 

Lepidolite 107 

Leucite 72 

Leucite basalt 169 

Leucite phonolite 162 

Leucite syenite 147 

Leucitite 169 

Light, nature of 13 

Limburgite 169 

Limestone 176 

Liparite 158 

Lithographic limestone 177 

Loess 175 

Magma 130 

Magmatic stoping 131 

Magnetite 71 

Malignite 153 

Mannebach 114 

Mapping 185 

Marble 182 



Mauna Loa 167 

Metamorphic rocks . 123, 172, 177 

Metamorphism 177 

Mica 105 

Mica plate 57, 59, 63 

Microcline 120 

Microcrystalline 128 

Micrometer 30 

Microscope 24 

Mineral determination. . . . 

33-67, 133 

Mineral description 68-122 

Mineralizers 126 

Mirror 28 

Missourite 154 

Monoclinic minerals, 48, 96-119 
Muscovite 106 

Natrolite 94 

Negative character 57-64 

Nephelite 83 

Nephelite basalt 169 

Nephelite syenite 147 

Newton's color scale 45 

Nicol prism 21, 24 

Norite 150 

Objective 28 

Oblique extinction 65 

Obsidian 159 

Obtuse bisectrix 60 

Ocular 30 

Oligoclase 121 

Olivine 92, 155, 166 

Olivine basalt 166 

Olivine diabase 166 

Oolitic limestone 177 

Opal 69 

Opaque minerals 34, 41 

Ophicalcite 183 

Ophitic texture 166 



196 



OPTICAL MINERALOGY AND PETROGRAPHY 



Optic axial plane 49 

Optic axis 26, 41, 51 

Optic normal 49 

Optic plane 49 

Optic section 42 

Optical character 56-64 

Optical mineralogy ' 9 

Orbicular granite 141 

Order of color 47 

Ordinary ray 18, 22, 42 

Orthoclase 119 

Orthorhombic minerals . . 48, 86 

Pantellerite 160 

Parallel extinction 65 

Pegmatite 139 

Penninite 109 

Pericline twinning 114 

Peridotite 155 

Perlite 160 

Petrogeny 9, 130 

Petrography 9, 10, 123 

Petrology 9 

Phenocrysts 127 

Phlogopite 108 

Phonolite 135, 162 

Phosphate 177 

Phyllite 182 

Pitchstone 159 

Pleochroism 66, 67 

Plutonic rocks 129, 138 

Polarization 21 

Polarizer 26, 41 

Polarizing microscope 24 

Porphyritic texture 127 

Positive character 57-64 

Primary minerals 125 

Principal optic section. .26, 42 

Prospecting 185 

Pumice 159 

Pyrite 70 



Pyroclastic rocks 170 

Pyroxene 89, 95 

Pyroxenite 157 

Pyrrhotite 70, 151 

Qualitative classification . . 123 
Quantitative classification . 123 
Quarter undulation mica 

plate 57, 59, 63 

Quartz 44, 79, 134 

Quartzite 181 

Quartz-sensitive tint 

58, 60, 61, 66 

Quartz wedge 47, 59, 62 

Recrystallization 178, 181 

Refraction 15-18, 21, 36 

Relief 36, 40 

Rhyolite 135, 158 

Riebeckite 104 

Rock classification 

Rogers 68 

Rosenbusch's law 128 

Rutile 75 

Sanadine 120 

Sand dunes 175 

Sandstone 174 

Scale of birefringence .... 50 

Scale of refringence 40 

Scapolite 76 

Schist 181 

Secondary minerals 125 

Sedimentary rocks 123, 172 

Sericite 107 

Serpentine 88, 183 

Serpentine rock 183 

Shale 174 

Shonkinite 153 

Siderite 81 

Slate . . 182 



INDEX 



197 



Soapstone 183 

Sodalite 73 

Spinel 71 

Stage 27 

Staurolite 87 

Steatite 183 

Syenite 145 

Talc 93 

Talus breccia 173 

Tephrite 168 

Tetragonal minerals ....41, 75 

Texture 125, 128 

Theralite 153 

Titanite 112 

Topaz 87 

Total reflection 16 

Tourmaline 84 

Trachydolerite 168 

Trachyte 161 

Trap 166 

Travertine 177 



Tremolite 102 

Triclinic minerals ..48, 120-122 

Tuff 171 

Twinning 35 

Undulatory extinction 66 

Uniaxial crystals 

14, 41, 51, 59, 01, 75 

Uralite 99 

Verde antique 183 

Volcanic agglomerate 170 

Volcanic breccia 171, 174 

Volcanic rocks 129, 158 

Wave-length 13, 21 

Werner, A. G 10 

Wernerite 76 

Winchell 40, 50 

Zircon 76 

Zirkel, Ferdinand 11, 12 

Zoisite . . Ill 



iiiiiliiffir ACIUTY 

AA 001204058 o 



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